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This CL adds some test coverage for animated scrolling. The test
verifies that mouse clicks on the scrollbar and mousewheel ticks
are animated by default. Verifying the curve type is not in scope.
This test was added because scroll animations were regressed by
crrev.com/c/2485659.

Change-Id: If44578745582fa7e69795886a4a3e1c80ecbd7ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521449
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: Olga Gerchikov <gerchiko@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#825180}