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This CL add system toast for reverse scrolling gesture changes.

 - If the user perform wrong gestures twice, a system toast will show up
 to indicate the right gestures.

 - If the toast shows at the fist time, it will disappear until the user
 perform the right gesture. Otherwise, it will shows for 2500ms.

Bug: 1107183

Change-Id: I1fd3fb9f0c898ca15cae2de87df6e89c7b62aecc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417242
Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Reviewed-by: Jun Mukai <mukai@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809431}