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This cl resolves the regression that is caused by the following cl:
https://chromium-review.googlesource.com/c/chromium/src/+/1313270

The previous cl calls UnregisterFlingSchedulerObserver() twice when a
fling gets cancelled: once from gesture_event_queue while handling the
generated GSE and once from FlingScheduler::DidStopFlingingOnBrowser.

When there is only one active fling and the fling is not interrupted by
another scroll/fling this works fine since the second Unregister call
does nothing. However sometimes when a new fling replaces an active fling
(e.g. starting a fling in the opposite direction of the currently active
fling) the second Unregister call unregisters the observer that is added
for the new fling. Since the observer is unregistered for the new fling
no ProgressFling will get called till the user does a third fling.
on the first ProgressFling call for the third fling, the controller tries
to cancel the second fling and calls unregister which unregisters the
observer for the third fling and the same thing happens for the rest of
the flings from now on.

This cl fixes the issue by not calling UnregisterFlingSchedulerObserver()
in FlingScheduler::DidStopFlingingOnBrowser since it will get called once
the generated GSE of the fling cancel is processed.

Bug: 901831
Change-Id: I4d52f512ea38764fc64c580f6cac5ed4a32c1bee
Reviewed-on: https://chromium-review.googlesource.com/c/1327145Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606606}