[Extensions] Add --enable-loading-extensions-on-demand switch

Runtime switch to protect the implementation of on demand loading of
extensions ("lazy load") until it's done, when we change the semantics
to be --disable.

In the release this feature debuts, we keep the disable switch to allow
QA disable if needed. After we succesfully ship one stable Crosswalk
with the feature (it will be Crosswalk 2), the current plan is to remove
the switch, because we don't intend to keep the codepath for the
non-lazy behavior.
Merge pull request #909 from cmarcelo/load-on-demand-switch

[Extensions] Add --enable-loading-extensions-on-demand switch
[Android] The first step to fix the resource id generation for core library.

Currently the resource ids for org.chromium.ui and org.chromium.content
are not generated so the web app will crash when running. Because
the container can't get the resources and their ids.
This patch is the first step to copy necessary resources from these two
targets to xwalk core library. The next step is to generate the correct
R.java.

BUG=#862
Merge pull request #906 from yongsheng/862

[Android] The first step to fix the resource id generation for core libr...
Bump version to 2.31.19.0
Add application extension and getManifest(), getMainDocument() API support

The application extension is an internal extension to handle application level
APIs(e.g., runtime events, manifest, package management, etc.).

In this commit two APIs is added, one is xwalk.application.getManifest(see
http://developer.chrome.com/apps/runtime.html#method-getManifest), the other is
xwalk.application.getMainDocument(see
http://developer.chrome.com/apps/runtime.html#method-getBackgroundPage).

We use asynchronous calls for both of these two APIs. getManifest() will
retrieve current running application's manifest dictionary value and post the
value to renderer. getMainDocument() utilizes ApplicationNativeModule to get
RenderView from the main document's routing ID, then return its main frame
global JS object.
Add application browser test base class and application API browser test

ApplicationBrowserTest is a base class for application browser test. Currently
it only provides Runtime status check. It will support other application test
utilities after shared browser process model is supported.

In this commit getManifest() and getMainDocument() API tests are added.