aura: Start using EventPrcessor interface for event dispatch.

Start converting the RootWindowHostDelegate implementations for event-dispatch
use the EventProcessor interface under-the-hood to allow easy transition to the
new dispatch process.

This patch makes this change for Key and Scroll events.

BUG=318879
R=ben@chromium.org

Review URL: https://codereview.chromium.org/113283005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240683 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240683