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By changing the test to use promises, assertion checks happen after giving the
scroll enough time to get processed.

TEST=fast/events/wheel/mouse-wheel-scroll-latching.html,
virtual/wheelscrolllatching/fast/events/wheel/mouse-wheel-scroll-latching.html

Bug: 735130
Change-Id: I1c13bf7eb4be88be942cea40e0bd3228ea2830d9
Reviewed-on: https://chromium-review.googlesource.com/591771
Commit-Queue: Sahel Sharifymoghaddam <sahel@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490793}