Use CancellationFlag instead of Lock in PostHelper

XWalkExtensionThreadedRunner have a helper object that runs jobs in
another thread. When threaded runner goes down, we want to
invalidate (cancel) that helper object.

This commit change from using a Lock and changing the runner_ value
inside the helper to simply use CancellationFlag (which boils down to an
atomic operated value).
[Devtools] Show right message when App is inspected already.
To correct a HTML/CSS coding error in Devtools discovery page
[Android] Create the file of parsing json-format manifest file.

This file is created to parse json-format manifest configuration files and will
be integrated with packaging tools to generate platform specific manifest files.
1 Parse the json-format manifest file and decomposed into fields.
2 Raise the related exceptions when there are parsing errors or key errors in
the json-format manifest file.
3 Provide the field interfaces required by packaging tool (e.g. make_apk.py).
4 Show the parsed fields and the corresponding values for debugging.
Refactor startup flags and enable viewport and touch feedback

The refactor makes the code similar to how it is done for Chrome for
Android and enables viewport (incl required fixed layout) and feedback
on touch.

It has been tested on desktop and works fine with the following:

xwalk "data:text/html,<meta name='viewport' content='width=320'>" \
  "<body><div style='width:300px; background-color:red;'> .</div></body>"
Merge pull request #586 from ShDavidliu/master

[Android] Parse json-format manifest file and provide the output fields
[Android] Integrate extension system with XWalkExtensionAndroid.

 * Enable XWalkCoreExtensionBridge as a bridge.
 * Implement unregisiter method for android xwalk extension.
 * Remove API version from XWalkExtension class.
 * Remove the code stub of sample extension.
Merge pull request #651 from kenchris/viewport-mode

Refactor startup flags and enable viewport and touch feedback
Merge pull request #660 from gaochun/extension_management

Integrate XWalk extension system with XWalkExtensionAndroid.
[Android] Remove unused build target xwalk_runtime_lib_apk_extension.

Since the js_api folder in assets is removed, the build target will not be
used any more.
Merge pull request #668 from gaochun/otcshare_master

[Android] Remove the unused build target xwalk_runtime_lib_apk_extensio...
[Android] Copy extension related Java files for xwalk core library

This library should include all core Java files. Newly-added Java
files for extension system should also be copied to avoid Java class
missing error.
Merge pull request #671 from yongsheng/master

[Android] Copy extension related Java files for xwalk core library
Merge pull request #576 from PeterWangIntel/master

[Devtools] Show right message when App is inspected already.
Merge pull request #663 from jeez/eph-channel

[Extensions] Create the EP <-> RP IPC Channel
Merge pull request #587 from cmarcelo/cancellation-flag

Use CancellationFlag instead of Lock in PostHelper
[Extensions] Add an external extensions XWalkExtensionClient to RendererController

XWalkExtensionClient::Initialize(IPC::Sender*) was added so we can:
  1- create a XWalkExtensionClient;
  2- initialize a SyncChannel using the Client as its listener;
  3- initialize the Client by passing the SyncChannel as its Sender.

This change follows exactly how XWalkExtensionServer is designed nowadays.
[Extensions] Don't register external extensions in BP if using EP