Adding compositor callbacks to RenderWidgetHostViewAndroid

The coupling between the compositor and the RenderWidgetHostViewAndroid (RWHVA)
is weak.  This causes problem such as when RWHVA needs to act on a compositor
commit (as in the current DidCommitFrameData callback).

We create a WindowAndroidObserver class and add a list of observers to the
android window, which then forwards the callbacks from the compositor to the
observers.  

android= https://gerrit-int.chromium.org/#/c/44978/

BUG=273250

Review URL: https://codereview.chromium.org/26753005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236598 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236598