NOT READY FOR REVIEW - Add support for custom animation timers.

ui/compositor/layer_animator.h

Index: ui/compositor/layer_animator.h
diff --git a/ui/compositor/layer_animator.h b/ui/compositor/layer_animator.h
index 6c24008cb0d7698e1cb2ecca9d43bd3de14a8244..37e182b743b93e0bd69ee0c08a003e22de2bfa07 100644
--- a/ui/compositor/layer_animator.h
+++ b/ui/compositor/layer_animator.h
@@ -29,6 +29,7 @@ class Layer;
 class LayerAnimationSequence;
 class LayerAnimationDelegate;
 class LayerAnimationObserver;
+class LayerAnimationTimer;
 class ScopedLayerAnimationSettings;
 
 // When a property of layer needs to be changed it is set by way of
@@ -193,6 +194,13 @@ class COMPOSITOR_EXPORT LayerAnimator
     return disable_animations_for_test_;
   }
 
+  // Takes ownership of the given timer.
+  // FIXME(vollick): Once we've written a TestLayerAnimationTimer than can be
+  // used to advance animations, we can probably get rid of last_tick_time() and
+  // all the code in the layer animation tests that advances animations using
+  // AnimationContainerElement::Step.
+  static void SetAnimationTimer(LayerAnimationTimer* timer);
+
  protected:
   virtual ~LayerAnimator();