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This CL enable the feature of reverse scrolling gestures.

 - Enable reverse scrolling gesture feature.
 - Get rid of the notification for overview gestures.
 - Add toast for the gestures entering overview when the natural
 scrolling is OFF.

Bug: 1107183
Change-Id: Ibbabca1b5deb5b37c8b697ff732f5312519e3adc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510350Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824219}