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.