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This CL adds a flag for the reverse scrolling features:

- kReverseScrollGestures: enable reverse scroll gestures

Bug: 1107183
Change-Id: Id1f918bacbca2726300927e7bf0e680b9237053b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419922
Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814922}