Reset gesture handling state on TransferEvents

This is phase-2 of tab-dragging with touch on Mash. Gesture
recognition keeps a flag to suppress emitting gesture events
once one of its touch event in the current event stream is
handled by someone else.

Without this change, some gesture events during tab-dragging
could be prevented on Mash, as described in
https://docs.google.com/document/d/1AKeK8IuF-j2TJ-2sPsewORXdjnr6oAzy5nnR1zwrsfc/edit#

BUG=867074
TEST=manually

Change-Id: I0186e58be2c3c42cc1070acb7a46e878b2b75981
Reviewed-on: https://chromium-review.googlesource.com/c/1247032
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596787}
7 files changed