Attempt to fix the crash in blink Scroll Animator.

https://crrev.com/1695763003 didn't fix the crash. Maybe the crash is because
the delegate is actually null but the call succeeds and eventually results in
a crash.

Another speculation is that ScrollAnimatorCompositorCoordinator gets garbage
collected by oilpan, but the player doesn't know about it. In that case, the
fix would be to use USING_PRE_FINALIZER and clear the player in dispose().

BUG=586400

Review URL: https://codereview.chromium.org/1714663002

Cr-Commit-Position: refs/heads/master@{#377005}