Add a flag for dispatching touch events to "SyntheticGestureController"

In Devtool Protocol Input.dispatchTouchEvent, we tried to dispatch
touch events to "SyntheticGestureController" to simulate touch events
close to real OS events, but there are some problems of converting the
synthetic touch events' coordinates correctly while the events are sent
from browser to renderer on all platforms. So we put this feature
behind a bug and disable it until we fix all the problems.

Bug: 893480
Change-Id: I86d4a4ebbbe03ee928c325fb9037ba839623e913
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506939
Commit-Queue: Ella Ge <eirage@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639148}
4 files changed