[Android] Set DIP scale and refine the parameter.

This patch is to set the DIP scale in the construction of XWalkContent
and refine the parameter of onScaleChanged().
[Android] Add test case for OnScaleChanged.

This patch is to add test case for OnScaleChanged().
Enable Alsa support

Bug=XWALK-2262
GetUserAgent shouldn't have Mobile for IVI

Bug = XWALK-2317
[REG] Fixed regression error that NaCl module failed to load

When rolling Chromium to version 37.0.2062.76, the interface
content::ContentMainDelegate::ZygoteStarting has been changed,
which makes it fails to call the old implementation in
xwalk::XWalkMainDelegate. Reimplemented it as indicated by chrome.

Also according to patch 1d7be68, nacl::NaClHelper is introduced
with the refactor of MessageChannel to control the in-process
and out-of-process message queue behavior, which should be created
inside xwalk::XWalkContentRendererClient::RenderFrameCreated when
NaCl is enabled.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2297
Merge pull request #2322 from qjia7/alsa

Enable Alsa support
Merge pull request #2313 from qjia7/useragent

GetUserAgent shouldn't have Mobile for IVI
Revert "Enable Alsa support"
Merge pull request #2334 from crosswalk-project/revert-2322-alsa

Revert "Enable Alsa support"
Merge pull request #2332 from jondong/nacl-load-regression

[REG] Fixed regression error that NaCl module failed to load
[Android][SysApps] Add Event type for DeviceCapabilities.

Add SystemDisplayEvent and SystemStorageEvent type.

BUG=XWALK-1301
Merge pull request #2329 from legendlee1314/fixbug

[Android][SysApps] Add Event type for DeviceCapabilities.
[Android][SysApps] Make StorageUnit.id and DisplayUnit.id be type of string.

BUG=XWALK-2354
Merge pull request #2314 from wuhengzhi/onScaleChanged_test

[Android] Set DIP scale and add test case for onScaleChanged.
Merge pull request #2324 from legendlee1314/cleanfails

[Android][SysApps] Make StorageUnit.id and DisplayUnit.id be type of string.
[Android] Enable reflection layer generator

Add a python script to generating code for classes in
org.xwalk.core from classes in org.xwalk.core.internal
which need to be exposed in Embedding API.

For each class of org.xwalk.core.internal which needs to be
exposed in Embedding API.
A bridge class and a wrapper class will be created for it.
The bridge class is a subclass of internal class and maintains
a wrapper object of this type.
A wrapper class is in org.xwalk.core and be exposed in Embedding
API, it maintains an instance of bridge object as well.
Wrapper and bridge talks to each other via pure reflection. So
they don't have to be in the same package in future.

TODO: JavascriptInterface class needs to be placed in wrapper and
pass the class object to internal for recognition of javascript
interface.

BUG Related: https://crosswalk-project.org/jira/browse/XWALK-1638