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The change to accessibility_focus_ring_layer.cc prevents an endless loop if
the input ring is all the same point.

The change to accessibility_focus_ring_controller.cc addresses the underlying
cause - the timestamp we get from CompositorAnimationObserver::OnAnimationStep
is not guaranteed to be later than the most recent focus time (in practice,
it's up to 0.002 s earlier on an ARM Chromebook). A negative delta time
turned into NaN when we computed pow(fraction, 0.3) and that led to the
input ring being all zeros, triggering the issue above.

BUG=427649

Review URL: https://codereview.chromium.org/693623003

Cr-Commit-Position: refs/heads/master@{#302178}