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 91d812c2558c2f646b638151ffcf4736e7e3ef01..10e8a199666a4f21d517d1401a55ba5ec733c65d 100644
--- a/cc/trees/property_tree_builder.cc
+++ b/cc/trees/property_tree_builder.cc
@@ -448,7 +448,8 @@ 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;
   }
 
@@ -619,8 +620,8 @@ 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;
 
   node->scrolls = is_scrollable;
   node->should_be_snapped = is_snapped;
@@ -1092,7 +1093,8 @@ bool AddEffectNodeIfNeeded(
   data_for_children->effect_tree_parent = effect_tree.Insert(node, parent_id);
   int node_id = data_for_children->effect_tree_parent;
   layer->SetEffectTreeIndex(node_id);
-  data_for_children->property_trees->effect_id_to_index_map[layer->id()] =
+  data_for_children->property_trees
+      ->layer_id_to_effect_node_index[layer->id()] =
       data_for_children->effect_tree_parent;
 
   std::vector<std::unique_ptr<CopyOutputRequest>> layer_copy_requests;
@@ -1179,7 +1181,8 @@ 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) {