cc : Track opacity animation changes on effect tree

cc/trees/mutator_host_client.h

Index: cc/trees/mutator_host_client.h
diff --git a/cc/trees/mutator_host_client.h b/cc/trees/mutator_host_client.h
index 1e1ec5e15012bfc2df7d01aa14770c3ea25a870c..b14871b2ee3a3c3dbf2297f5d84d5eba119150f2 100644
--- a/cc/trees/mutator_host_client.h
+++ b/cc/trees/mutator_host_client.h
@@ -18,6 +18,8 @@ using ElementId = int;
 
 enum class ElementListType { ACTIVE, PENDING };
 
+enum class AnimationChangeType { POTENTIAL, RUNNING, BOTH };
+
 class MutatorHostClient {
  public:
   virtual bool IsElementInList(ElementId element_id,
@@ -45,6 +47,11 @@ class MutatorHostClient {
       ElementListType list_type,
       bool is_animating) = 0;
 
+  virtual void ElementOpacityIsAnimatingChanged(ElementId element_id,
+                                                ElementListType list_type,
+                                                AnimationChangeType change_type,
+                                                bool is_animating) = 0;
+
   virtual void ScrollOffsetAnimationFinished() = 0;
   virtual gfx::ScrollOffset GetScrollOffsetForAnimation(
       ElementId element_id) const = 0;