[Android] Consolidate touch handling code in ContentViewGestureHandler

Unify the code that dispatches touch events to native, making it easier to
follow touch event flow through the pipeline.  Also guard against nested loops
for touch dispatch, triggered by synchronous acks when there are multiply
enqueud touch events.

Review URL: https://codereview.chromium.org/119333002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242124 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242124