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 509 matching lines @@
 
 class CC_EXPORT PropertyTrees final {
  public:
   PropertyTrees();
   PropertyTrees(const PropertyTrees& other) = delete;
   ~PropertyTrees();
 
   bool operator==(const PropertyTrees& other) const;
   PropertyTrees& operator=(const PropertyTrees& from);
 
-  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 };
 
   std::vector<int> always_use_active_tree_opacity_effect_ids;
   TransformTree transform_tree;
   EffectTree effect_tree;
   ClipTree clip_tree;
   ScrollTree scroll_tree;
   bool needs_rebuild;
   bool non_root_surfaces_enabled;
   // Change tracking done on property trees needs to be preserved across commits
@@ skipping 69 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_