Clarify property tree id-to-index map names.

cc/trees/property_tree_builder.cc

Index: cc/trees/property_tree_builder.cc
diff --git a/cc/trees/property_tree_builder.cc b/cc/trees/property_tree_builder.cc
index 8cac600ba14507e71f3a39f3101c7525bd0d25c9..763d50016bbe63119fb5fe2117a6b327cdf3b23f 100644
--- a/cc/trees/property_tree_builder.cc
+++ b/cc/trees/property_tree_builder.cc
@@ -462,7 +462,8 @@ void AddClipNodeIfNeeded(const DataForRecursion<LayerType>& data_from_ancestor,
 
     data_for_children->clip_tree_parent =
         data_for_children->property_trees->clip_tree.Insert(node, parent_id);
-    data_for_children->property_trees->clip_id_to_index_map[layer->id()] =
+    data_for_children->property_trees
+        ->layer_id_to_clip_node_index[layer->id()] =
         data_for_children->clip_tree_parent;
   }
 
@@ -633,8 +634,8 @@ bool AddTransformNodeIfNeeded(
   TransformNode* node =
       data_for_children->property_trees->transform_tree.back();
   layer->SetTransformTreeIndex(node->id);
-  data_for_children->property_trees->transform_id_to_index_map[layer->id()] =
-      node->id;
+  data_for_children->property_trees
+      ->layer_id_to_transform_node_index[layer->id()] = node->id;
 
   // For animation subsystem purposes, if this layer has a compositor element
   // id, we build a map from that id to this transform node.
@@ -1105,8 +1106,8 @@ bool AddEffectNodeIfNeeded(
   int node_id = effect_tree.Insert(node, parent_id);
   data_for_children->effect_tree_parent = node_id;
   layer->SetEffectTreeIndex(node_id);
-  data_for_children->property_trees->effect_id_to_index_map[layer->id()] =
-      node_id;
+  data_for_children->property_trees
+      ->layer_id_to_effect_node_index[layer->id()] = node_id;
 
   // For animation subsystem purposes, if this layer has a compositor element
   // id, we build a map from that id to this effect node.
@@ -1199,7 +1200,8 @@ void AddScrollNodeIfNeeded(
         node.main_thread_scrolling_reasons;
     data_for_children->scroll_tree_parent_created_by_uninheritable_criteria =
         scroll_node_uninheritable_criteria;
-    data_for_children->property_trees->scroll_id_to_index_map[layer->id()] =
+    data_for_children->property_trees
+        ->layer_id_to_scroll_node_index[layer->id()] =
         data_for_children->scroll_tree_parent;
 
     if (node.scrollable) {