Also without applying rail behavior to GestureScrollBegin, we will have situations where delta_{x,y}_hint and delta_{x,y} will have a different value between GSB and first GSU.

Unless there is any compelling reason to do this I think we should ensure these match.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a1d514a3e8651084af4a363c1e78a211deaa6edb

commit a1d514a3e8651084af4a363c1e78a211deaa6edb
Author: Sandra Sun <sunyunjia@chromium.org>
Date: Wed Aug 09 15:07:45 2017

Rail the delta_hint in GestureScrollBegin

Currently for the gesture events sent from the browser, we only rail the delta
in GestureScrollUpdate. We are not railing the delta_hint in
GestureScrollBegin. This could cause some complications when handling
ScrollBoundaryBehavior.

We should also rail the delta_hint in GestureScrollBegin. Specifically, in
GestureProvider, we should generate GestureScrollBegin event by using the
updated distance rather than raw_distance.

Bug:  751795 
Change-Id: I94270a80a297e42ef188c9e0efeb577114681516
Reviewed-on: https://chromium-review.googlesource.com/598770
Commit-Queue: Sandra Sun <sunyunjia@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492983}
[modify] https://crrev.com/a1d514a3e8651084af4a363c1e78a211deaa6edb/ui/aura/gestures/gesture_recognizer_unittest.cc
[modify] https://crrev.com/a1d514a3e8651084af4a363c1e78a211deaa6edb/ui/events/gesture_detection/gesture_provider.cc
[modify] https://crrev.com/a1d514a3e8651084af4a363c1e78a211deaa6edb/ui/events/gesture_detection/gesture_provider_unittest.cc