Clarify property tree id-to-index map names.

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..4b08d0a15a2681d4b8aea7dc63a3e22195a43262 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -527,10 +527,10 @@ class CC_EXPORT PropertyTrees final {
   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;