cc: Use Element Id to Record Animation Changes

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 8ccd09b46fe4e1e3fc1cbcdd82b946255da771cd..4f37be2fc35f9d122e5c86471cd924236f06ef31 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -510,9 +510,11 @@ class CC_EXPORT LayerTreeImpl {
 
   std::unordered_map<ElementId, int, ElementIdHash> element_layers_map_;
 
-  std::unordered_map<int, float> opacity_animations_map_;
-  std::unordered_map<int, gfx::Transform> transform_animations_map_;
-  std::unordered_map<int, FilterOperations> filter_animations_map_;
+  std::unordered_map<ElementId, float, ElementIdHash> opacity_animations_map_;

[wkorman, 2017/03/20 21:59:57]

Thoughts on naming these xx_to_yy? We did that for recently added
PropertyTrees::element_id_to_xxx_node_index for example. It can make the calling
code easier to follow since it's more obvious what we're mapping from/to.

So here would be something like element_id_to_opacity, element_id_to_transform,
element_id_to_filter_ops.

+  std::unordered_map<ElementId, gfx::Transform, ElementIdHash>
+      transform_animations_map_;
+  std::unordered_map<ElementId, FilterOperations, ElementIdHash>
+      filter_animations_map_;
 
   // Maps from clip layer ids to scroll layer ids.  Note that this only includes
   // the subset of clip layers that act as scrolling containers.  (This is