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This UIViewPropertyAnimator subclass records whether animations have
been added and early returns before attempting to start an animator that
has no animation blocks.  This is necessary because the default
implementations of |-startAnimation| and |-startAnimationAfterDelay:|
throw exceptions if there are no animations.

This CL also updates FullscreenAnimator to be optional, as there is no
guarantee that FullscreenControllerObservers will add animation blocks
to FullscreenAnimators.

Bug: 819623
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I1e73a7163f5bd9b82cf4ff693b290464ecec63cd
Reviewed-on: https://chromium-review.googlesource.com/953793Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541909}