Fix system freeze in accessibility focus ring code.

chrome/browser/chromeos/ui/accessibility_focus_ring_controller.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chromeos/ui/accessibility_focus_ring_controller.h"
 
 #include "ash/display/display_controller.h"
 #include "ash/shell.h"
 #include "base/logging.h"
 #include "chrome/browser/chromeos/ui/focus_ring_layer.h"
@@ skipping 279 matching lines @@
 void AccessibilityFocusRingController::OnAnimationStep(
     base::TimeTicks timestamp) {
   if (rings_.empty())
     return;
 
   CHECK(compositor_);
   CHECK(!rings_.empty());
   CHECK(!layers_.empty());
   CHECK(layers_[0]);
 
+  // It's quite possible for the first 1 or 2 animation frames to be
+  // for a timestamp that's earlier than the time we received the
+  // focus change, so we just treat those as a delta of zero.
+  if (timestamp < focus_change_time_)
+    timestamp = focus_change_time_;
+
   base::TimeDelta delta = timestamp - focus_change_time_;
   base::TimeDelta transition_time =
       base::TimeDelta::FromMilliseconds(kTransitionTimeMilliseconds);
   if (delta >= transition_time) {
     layers_[0]->Set(rings_[0]);
     compositor_->RemoveAnimationObserver(this);
     compositor_ = NULL;
     return;
   }
 
   double fraction = delta.InSecondsF() / transition_time.InSecondsF();
 
   // Ease-in effect.
   fraction = pow(fraction, 0.3);
 
   layers_[0]->Set(AccessibilityFocusRing::Interpolate(
       previous_rings_[0], rings_[0], fraction));
 }
 
 }  // namespace chromeos