Clarify property tree id-to-index map names.

cc/trees/property_tree.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_PROPERTY_TREE_H_
 #define CC_TREES_PROPERTY_TREE_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 511 matching lines @@
  public:
   PropertyTrees();
   PropertyTrees(const PropertyTrees& other) = delete;
   ~PropertyTrees();
 
   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;
+  std::unordered_map<int, int> layer_id_to_transform_node_index;
+  std::unordered_map<int, int> layer_id_to_effect_node_index;
+  std::unordered_map<int, int> layer_id_to_clip_node_index;
+  std::unordered_map<int, int> layer_id_to_scroll_node_index;
   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;
@@ skipping 80 matching lines @@
   DrawTransforms& GetDrawTransforms(int transform_id, int effect_id) const;
   DrawTransformData& FetchDrawTransformsDataFromCache(int transform_id,
                                                       int effect_id) const;
 
   PropertyTreesCachedData cached_data_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_PROPERTY_TREE_H_