Clarify property tree id-to-index map names.

cc/input/scrollbar_animation_controller_linear_fade.cc

 // Copyright 2012 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.
 
 #include "cc/input/scrollbar_animation_controller_linear_fade.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/time/time.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
@@ skipping 45 matching lines @@
 }
 
 void ScrollbarAnimationControllerLinearFade::ApplyOpacityToScrollbars(
     float opacity) {
   for (ScrollbarLayerImplBase* scrollbar : Scrollbars()) {
     if (!scrollbar->is_overlay_scrollbar())
       continue;
     PropertyTrees* property_trees =
         scrollbar->layer_tree_impl()->property_trees();
     // If this method is called during LayerImpl::PushPropertiesTo, we may not
-    // yet have valid effect_id_to_index_map entries as property trees are
-    // pushed after layers during activation. We can skip updating opacity in
-    // that case as we are only registering a scrollbar and because opacity will
-    // be overwritten anyway when property trees are pushed.
+    // yet have valid layer_id_to_effect_node_index entries as property trees
+    // are pushed after layers during activation. We can skip updating opacity
+    // in that case as we are only registering a scrollbar and because opacity
+    // will be overwritten anyway when property trees are pushed.
     if (property_trees->IsInIdToIndexMap(PropertyTrees::TreeType::EFFECT,
                                          scrollbar->id())) {
       property_trees->effect_tree.OnOpacityAnimated(
           scrollbar->CanScrollOrientation() ? opacity : 0,
-          property_trees->effect_id_to_index_map[scrollbar->id()],
+          property_trees->layer_id_to_effect_node_index[scrollbar->id()],
           scrollbar->layer_tree_impl());
     }
   }
 }
 
 }  // namespace cc