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

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

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

chrome/browser/chromeos/ui/accessibility_focus_ring_controller.cc

@@ -297,6 +297,12 @@ void AccessibilityFocusRingController::OnAnimationStep(
   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);

chrome/browser/chromeos/ui/accessibility_focus_ring_layer.cc

@@ -41,13 +41,13 @@ SkPath MakePath(const AccessibilityFocusRing& input_ring,
     do {
       prev_index = (prev_index + 35) % 36;
       prev = input_ring.points[prev_index];
-    } while (prev.x() == p.x() && prev.y() == p.y());
+    } while (prev.x() == p.x() && prev.y() == p.y() && prev_index != i);
 
     int next_index = i;
     do {
       next_index = (next_index + 1) % 36;
       next = input_ring.points[next_index];
-    } while (next.x() == p.x() && next.y() == p.y());
+    } while (next.x() == p.x() && next.y() == p.y() && next_index != i);
 
     gfx::Point delta0 = gfx::Point(sign(p.x() - prev.x()),
                                    sign(p.y() - prev.y()));