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The previous CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1102729
was reverted. The reason is that in the test we have two gesture scroll,
and the second gesture starts before the first TapDown finishes.

This CL solve the problem. PS#1 is exactly the same as the previous CL
which was reverted, so that it is easier for review.

TBR=creis@chromium.org

Bug: 851644
Change-Id: Ife9bf8f9a37892380e6587b80bcc03366cc0e80d
Reviewed-on: https://chromium-review.googlesource.com/1151094
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579074}