cc : Track opacity animation changes on effect tree

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 63208b4e578c46ffa5a38107a9c5cfdb7cb290a3..934b889759df4e6b14c8dafd1cd202f4f924f996 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -379,6 +379,9 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
       ElementId element_id,
       ElementListType list_type,
       bool is_animating) override;
+  void ElementOpacityIsCurrentlyAnimatingChanged(
+      ElementId element_id,
+      bool is_currently_animating) override;
   void ScrollOffsetAnimationFinished() override {}
   gfx::ScrollOffset GetScrollOffsetForAnimation(
       ElementId element_id) const override;