[Extensions] Flip flag to enable loading extensions on demand

Now that it works, we can flip the flag to be enabled by default.
Merge pull request #976 from vcgomes/flip-load-on-demand-flag

[Extensions] Flip flag to enable loading extensions on demand
Bump version to 2.31.24.0
Merge branch 'master' of github.com-crosswalk:crosswalk-project/crosswalk
[Android] Adjust the parameters for extension's two methods for reflection.

The onMessage and onSyncMessage are changed due to the support for multiple
instances for extension mechanism. However, the Java extension mechanism
doesn't change its parameters when looking up the methods for external
extensions. The unit test for external extensions will be added soon.

BUG=
[Extension] Fix build failure on Windows

Use '\' for the product_dir on Windows in external_extension_sample.gyp.
Otherwise, the build system can't handle the path well in some cases,
e.g. creating or deleting them.
It fails the windows build when it starts from zero.
Merge pull request #969 from wang16/fix_windows

[Extension] Fix build failure on Windows
Merge pull request #978 from yongsheng/multiinstance

[Android] Adjust the parameters for extension's two methods for reflecti...
[Android] Add the interface of loadAssetFile and onTitleChanged.

Add the interfaces in embedded and shared mode.
loadAssetFile: It is needed when we put the test files(eg. html) under
the test assets.
onTitleChanged: Implement a callback function in the test client,
transfer it to XWalkCoreProvider, then set it to the XWalkWebChromeClient,
In the XWalkWebChromeClient,'s callback, invoke the callback functions
which we defined, notify the test client about the title changed.
Move most of the util codes to a utility folder for reuse.
[Android] Add the dependency about test apk.

On the buildbot, we need to generate the
XWalkRuntimeClientEmbeddedTest.apk and XWalkRuntimeClientTest.apk
before the instrumentation test.
Merge pull request #970 from wuhengzhi/runtime_client

[Android] Add the interface of loadAssetFile and onTitleChanged.
Merge pull request #985 from wuhengzhi/xwalk

[Android] Add the dependency about test apk.
[Android] Disabled two test cases about ExtensionBroadcast and ExtensionEchoMultiFrames.

Due to these two test cases failed on buildbot, disabled them temporally.
They will be enabled later.
Merge pull request #989 from wuhengzhi/runtime_client

[Android] Disabled two test cases about ExtensionBroadcast and Extension...
[Extensions] Fix race condition in Extensions registering.

Nowadays we are facing a race condition between the moment
we get the first XWalkExtensionRendererController::DidCreateScriptContext()
for a given RenderProcess and the moment we register the external extensions
at XWalkExtensionRendererController's external_extensions_client_.

This patch fixes this by inverting the registration order. Now
during the Client's initialization we synchronously ask the Server
for the extensions to register.

In addition to that, after XWalkExtensionProcessMsg_RegisterExtensions
we have added a WaitableEvent so we can guarantee all external
extensions have been loaded way before doing anything else on
the BrowserProcess (more specifically at
XWalkExtensionService::OnRenderProcessCreated()).
TEMP: disable loading extensions on Demand to test Trybots