Fix system freeze in accessibility focus ring code.

Fix system freeze in accessibility focus ring code.

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
Committed: https://crrev.com/822d339a7a1c626781180fd233781de7d760eef1
Cr-Commit-Position: refs/heads/master@{#302178}

[dmazzoni, 2014-10-30 21:49:00]

dmazzoni@chromium.org changed reviewers:
+ xiyuan@chromium.org

[xiyuan, 2014-10-30 21:59:34]

lgtm

[dmazzoni, 2014-10-30 22:00:16]

The CQ bit was checked by dmazzoni@chromium.org

[commit-bot: I haz the power, 2014-10-30 22:00:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/693623003/1

[commit-bot: I haz the power, 2014-10-30 22:43:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-30 22:44:11]

Patchset 1 (id:??) landed as
https://crrev.com/822d339a7a1c626781180fd233781de7d760eef1
Cr-Commit-Position: refs/heads/master@{#302178}