cc : Add OnOpacityAnimated to effect tree

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index e8348462663420c8cacdd4407214991975fc40d6..57885eb15590c428e8246886e80685beed25e40a 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -3918,7 +3918,8 @@ void LayerTreeHostImpl::SetTreeLayerOpacityMutated(ElementId element_id,
 
   LayerImpl* layer = tree->LayerByElementId(element_id);
   if (layer)
-    layer->OnOpacityAnimated(opacity);
+    tree->property_trees()->effect_tree.OnOpacityAnimated(
+        opacity, layer->effect_tree_index(), tree);

[ajuma, 2016/08/18 18:49:26]

Can we get the effect tree index from the id to index map to remove the
dependency on |layer| altogether? Is the problem that we don't have a way to map
from element_id to layer id? If so, I think the id-to-index map should really be
mapping ElementId to index rather than layer id to index. (But that change can
happen in a separate CL.)

[jaydasika, 2016/08/18 19:57:52]

Acknowledged. Will do that in a separate CL.

 }
 
 void LayerTreeHostImpl::SetTreeLayerTransformMutated(