Build mapping from element id to transform/effect nodes.

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index c56126a05bb68a32b1da9c7ac5bce23d15083a0b..db53f85e5d6b043f6c7369164019a48ec2200f70 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -527,12 +527,25 @@ class CC_EXPORT PropertyTrees final {
   bool operator==(const PropertyTrees& other) const;
   PropertyTrees& operator=(const PropertyTrees& from);
 
+  // These maps map from layer id to the index for each of the respective
+  // property node types.
   std::unordered_map<int, int> transform_id_to_index_map;
   std::unordered_map<int, int> effect_id_to_index_map;
   std::unordered_map<int, int> clip_id_to_index_map;
   std::unordered_map<int, int> scroll_id_to_index_map;
   enum TreeType { TRANSFORM, EFFECT, CLIP, SCROLL };
 
+  // These maps allow mapping directly from a compositor element id to the
+  // respective property node. This will eventually allow simplifying logic in
+  // various places that today has to map from element id to layer id, and then
+  // from layer id to the respective property node. Completing that work is
+  // pending the launch of Slimming Paint v2 and reworking UI compositor logic
+  // to produce cc property trees and these maps.
+  std::unordered_map<ElementId, int, ElementIdHash>
+      element_id_to_effect_node_index;
+  std::unordered_map<ElementId, int, ElementIdHash>
+      element_id_to_transform_node_index;
+
   std::vector<int> always_use_active_tree_opacity_effect_ids;
   TransformTree transform_tree;
   EffectTree effect_tree;