aura: Dispatch mouse events using the EventProcessor interface.

This would be the last major patch for converting to the new event-dispatch
mechanism. Subsequent CLs will start removing the old code path (and
corresponding API where appropriate).

BUG=318879
R=ben@chromium.org

Review URL: https://codereview.chromium.org/136863005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245797 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245797