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This CL adds the left gradient zone during gesture drag and scrolling
animation.

Bug: 1004134
Change-Id: I134b0ea33ae06cbfe0974113ed08203d2b8c63cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1818118
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700450}