Code base of xwalk WebDriver, copied from ../src/chrome/test/chromedriver.
Invlove build target in gyp file and make base code to pass compiling.

(1) Involve target "xwalkdriver" and it's dependencies in xwalk_test.gypi
(2) Change hard-coded headers in source files, e.g.,
    #include "chrome/test/chromedriver/xx.h"
    is changed to
    #include "xwalk/test/chromedriver/xx.h"
To support "InitSession" function for xwalk.

The Chromedriver is generally a http server to translate http request (from
outside scripts) to websocket request of Devtools of Chrome.

The most of code inside of folder "server" can be reused. This code mainly
implement a simple http server that will call corresponding "command" to deal
with requests.

This commit changed the first command "InitSession" to adapt xwalk, as
following figure shows.

chromedriver_server.cc:    main
                             |
http_handler.cc:        HttpHandler
                             |
                   "CommandMapping commands[];"
                                         |
                             +-----------+-----
                             |           |
                        "InitSession"   ...
                             |
session_commands.cc:  ExecuteInitSession
                             |
                      InitSessionHelper
                             |
xwalk_launcher.cc:       LaunchXwalk
                             |
                             +-------------------+
                             |                   |
                     LaunchDesktopXwalk    LaunchAndroidXwalk
                             |                   |
                             +-----+             +-------------+
                             |     |             |             |
xwalk_finder.cc:         FindXwalk |            TODO           |
                                +--+---------+                 |
                                |            |                 |
xwalk_desktop_impl.cc:   XwalkDesktopImpl XwalkExistingImpl XwalkAndroidImpl
xwalk_android_impl.cc
xwalk_existing_impl.cc
Remove commands implemented by the Chrome extension.

These commands are: GetWindowPosition, SetWindowPosition, GetWindowSize,
SetWindowSize, MaximizeWindow, and Screenshot.
We leave "TDOD" in the interfaces and will implement soon.

Refer to below figure to check modified code.

chromedriver_server.cc:    main
                             |
http_handler.cc:        HttpHandler
                             |
                   "CommandMapping commands[];"
                                         |
                             +-----------+---------------+------------+--
                             |           |               |            |
                            ...    "Screenshot" "GetWindowPosition"  ...
                                         |               |
window_commands.cc:             ExecuteScreenshot        |
                                         |               |
session_commands.cc:                     |       ExecuteGetWindowPosition
                                         |               |
                                       TODO             TODO
Clear code of interfaces related with Android browser.

Since existent code doesn't work naturally by our investigation, so we leave it with "TODO".

Refer to below figure to check related code.
chromedriver_server.cc:    main
                             |
http_handler.cc:        HttpHandler
                             |
                   "CommandMapping commands[];"
                                         |
                             +-----------+-----
                             |           |
                        "InitSession"   ...
                             |
session_commands.cc:  ExecuteInitSession
                             |
                      InitSessionHelper
                             |
xwalk_launcher.cc:       LaunchXwalk
                             |
                             +-------------------+
                             |                   |
                     LaunchDesktopXwalk    LaunchAndroidXwalk
                             |                   |
                             +-----+             +-------------+
                             |     |             |             |
xwalk_finder.cc:         FindXwalk |            ...            |
                                +--+---------+                 |
                                |            |                 |
xwalk_android_impl.cc:         ...          ...          XwalkAndroidImpl
                                                               |
device_manager.cc:                                        DeviceManager
                                                               |
                                                            Device
                                                               |
adb_impl.h:                                                 AdbImpl
Add stub class for Tizen xwalk.

May invlove/modify other files when it's implemented.
Refer to to below figure.

chromedriver_server.cc:    main
                             |
http_handler.cc:        HttpHandler
                             |
                   "CommandMapping commands[];"
                                         |
                             +-----------+-----
                             |           |
                        "InitSession"   ...
                             |
session_commands.cc:  ExecuteInitSession
                             |
                      InitSessionHelper
                             |
xwalk_launcher.cc:       LaunchXwalk
                             |
                             +-------------------+
                             |                   |
                     LaunchDesktopXwalk    LaunchTizenXwalk
                             |                   |
                             +-----+             +-------------+
                             |     |             |             |
xwalk_finder.cc:         FindXwalk |            ...            |
                                +--+---------+                 |
                                |            |                 |
xwalk_tizen_impl.cc:           ...          ...          XwalkTizenImpl
                                                               |
                                                             TODO
Remove test scripts that cannot work with xwalkdriver naturelly.

We'll submmit a clean and working bunch of scripts soon.
Clean the code.

Including:
(1) Change variables named "*chrome*" to "*xwalk*".
(2) Correct lint warnings (some of them are caused by legacy code :( ).
(3) Change path name that including "chrome".
(4) Clean test scripts including Chrome-ralated stuffs.
(5) Rewrite README.txt.