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..49adaf8a870a426b34204e03aef2aa63856f0b6f 100644
--- a/cc/trees/mutator_host_client.h
+++ b/cc/trees/mutator_host_client.h
@@ -45,6 +45,10 @@ class MutatorHostClient {
       ElementListType list_type,
       bool is_animating) = 0;
 
+  virtual void ElementOpacityIsCurrentlyAnimatingChanged(
+      ElementId element_id,
+      bool is_currently_animating) {}
+
   virtual void ScrollOffsetAnimationFinished() = 0;
   virtual gfx::ScrollOffset GetScrollOffsetForAnimation(
       ElementId element_id) const = 0;