Cancel track autoscroll when thumb reaches the pointer.

This CL enables track autoscrolling to be aborted when the scrollbar
thumb reaches the pointer. Gesture based autoscrolling is done as a
scroll animation (as opposed to main thread autoscrolling which is
done by timer-based scroll updates). The idea here is to check the
position of the thumb at every frame and if it has crossed the pointer,
abort the scroll animation.

Bug: 978115
Change-Id: Ia0e387c255d808bfea6ef571bf2ce9ca591a19eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696343
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677442}