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Registering the FlingScheduler after processing a GFS causes a frame of
jank at the beginning of the fling. This is because the first OnAnimationStep
call after observer's registration might be missing and/or incorrectly have
the timestamp of the last begin frame before the observer's registration
rather than the first begin frame after the registration.

To avoid this issue, in this cl we register the observer on GSB instead of GFS
so that the potential missing/incorrect OnAnimationStep call passes by the time
that the GFS is processed. We unregister the observer on GSE instead of when
the fling stops since not all GSBs are followed by GFS events.

Bug: 882907
Test: GestureEventQueueWithCompositorEventQueueTest.FlingSchedulerOpbserverRegisteredOnGSB
Change-Id: I149469fce31c9146c7282bee5cba7152ee65b1ea
Reviewed-on: https://chromium-review.googlesource.com/c/1313270Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604707}