aura: Start updating aura to use EventProcessor for event dispatch.

Notable changes in this CL:
 * A WindowTargeter implementation for finding the target Window for an event
   (currently unused), with only a most basic test.
 * Make RootWindow a ui::EventProcessor.
 * Make RootWindowHostX11 a ui::EventSource.

Note that RootWindowHostX11 continues to dispatch events to the RootWindow using
RootWindowHostDelegate interface. This CL prepares much of the machinery for
switching to EventProcessor interface for event dispatch, but it is essentially a
no-op for now.

BUG=318879
R=ben@chromium.org

Review URL: https://codereview.chromium.org/108043005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239277 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239277