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Slow scrolling was not working properly because of an optimization I
tried to add to not process scroll events that barely change the grid
but it was dropping too many events resulting in small scrolls not
eventually scrolling the grid at all.

Fix a bug with fling and add a test for it. Fling was not working, due
to a bug where the grid wouldn't shift until the API returned no offset,
in which it would not shift anyways.

Test: Added test
Bug: 978112
Change-Id: Ib9544706975aa7adfa5163c07eb835c1b06708e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1802957
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697754}