This change adds a delay before Animator::BeginFrame calls its
delegate's OnAnimatorNotifyIdle. This is because under certain
workloads, such as our parent view resizing us, which is communicated
via viewport change events, we won't have a frame scheduled yet in the
animator, despite the fact that we will go on to schedule a frame once
the viewport event arrives.
In Fuchsia's resizing performance test, on our reference high end x86-64
hardware, the previous logic was resulting in a ~45ms garbage collection
right after the first frame of an animation.
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