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This CL makes it so that the CC animated scroll path latches only in
ScrollAnimatedBegin and prevents it from re-latching at all in
ScrollAnimated.

Since the targetted scrolling node can no longer change in
ScrollAnimated it no longer needs to return any value so the signature
of the method and some tests had to change.

Bug: 1016229
Change-Id: I9b3609de3f22dd7859bece5bb121c7ea752779a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980984
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727997}