Add flag to disable CancelAllTouches()

This flag is necessary for Blink team to test the new function that
cancels single touches.

Bug: 946868
Change-Id: I5799fdd3d39abff99d5d0c4e36424c65d4c505e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604382
Reviewed-by: Jia Meng <jiameng@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Thanh Nguyen <thanhdng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658875}
8 files changed