GitLab

- Add a ScopedAllow helper class that SystemEventRewriter allows
  event to pass through when there are instances of it;
- Use ScopedAllow helper to allow fake gesture events to pass
  through under mash;

Bug: 884360
Change-Id: I940398b0920505743ad7adbe08e12e1394fb55c0
Reviewed-on: https://chromium-review.googlesource.com/c/1292400Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605496}