Synchronize GestureRecognizer status between Ash and browser

This is phase-3 of tab-dragging with touch on Mash. This adds
GestureSynchronizer in ui/aura/mus, which observes GestureRecognizer
and sends several mojo methods so that the GestureRecognizer
instance in Ash can be synchronized.

BUG=867074
TEST=aura_unittests

Change-Id: I9993897e3da6c282ebfbc1aa599162255b5944d6
Reviewed-on: https://chromium-review.googlesource.com/c/1247033
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598130}
11 files changed