aura: Use EventPrcessor interface for dispatching touch events.

As with for key and scroll events, change the RootWindowHostDelegate implementation
to use EventProcessor interface for dispatching touch events under-the-hood.

BUG=318879
R=ben@chromium.org

Review URL: https://codereview.chromium.org/110483004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243306 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243306