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The completion blocks of the property animator is not guaranteed to be
called in the synchronously from |-finishAnimationAtPosition:|, so
despite previous safeguards put in place, the animator was still
sometimes deallocated when the state was active.  This CL updates
StopAnimating() to no longer perform property animator completion
blocks.

Bug: 793623
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I5d6d08c229d5fabe78f2ff79ab69560f79532d21
Reviewed-on: https://chromium-review.googlesource.com/835790Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525362}