Clarify property tree id-to-index map names.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 9d01bd653cb692523f0cfccf3bcc9f098ad0f097..5a06d2c4d3f39f99c9b8982ce5f286dfc38e8634 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -3898,7 +3898,7 @@ void LayerTreeHostImpl::SetTreeLayerFilterMutated(
   DCHECK(tree->property_trees()->IsInIdToIndexMap(
       PropertyTrees::TreeType::EFFECT, layer_id));
   const int effect_id =
-      tree->property_trees()->effect_id_to_index_map[layer_id];
+      tree->property_trees()->layer_id_to_effect_node_index[layer_id];
   if (effect_id != EffectTree::kInvalidNodeId)
     tree->property_trees()->effect_tree.OnFilterAnimated(filters, effect_id,
                                                          tree);
@@ -3914,7 +3914,7 @@ void LayerTreeHostImpl::SetTreeLayerOpacityMutated(ElementId element_id,
   DCHECK(tree->property_trees()->IsInIdToIndexMap(
       PropertyTrees::TreeType::EFFECT, layer_id));
   const int effect_id =
-      tree->property_trees()->effect_id_to_index_map[layer_id];
+      tree->property_trees()->layer_id_to_effect_node_index[layer_id];
   if (effect_id != EffectTree::kInvalidNodeId)
     tree->property_trees()->effect_tree.OnOpacityAnimated(opacity, effect_id,
                                                           tree);
@@ -3931,7 +3931,7 @@ void LayerTreeHostImpl::SetTreeLayerTransformMutated(
   DCHECK(tree->property_trees()->IsInIdToIndexMap(
       PropertyTrees::TreeType::TRANSFORM, layer_id));
   const int transform_id =
-      tree->property_trees()->transform_id_to_index_map[layer_id];
+      tree->property_trees()->layer_id_to_transform_node_index[layer_id];
   if (transform_id != TransformTree::kInvalidNodeId)
     tree->property_trees()->transform_tree.OnTransformAnimated(
         transform, transform_id, tree);
@@ -3953,9 +3953,9 @@ void LayerTreeHostImpl::SetTreeLayerScrollOffsetMutated(
   DCHECK(tree->property_trees()->IsInIdToIndexMap(
       PropertyTrees::TreeType::SCROLL, layer_id));
   const int transform_id =
-      tree->property_trees()->transform_id_to_index_map[layer_id];
+      tree->property_trees()->layer_id_to_transform_node_index[layer_id];
   const int scroll_id =
-      tree->property_trees()->scroll_id_to_index_map[layer_id];
+      tree->property_trees()->layer_id_to_scroll_node_index[layer_id];
   if (transform_id != TransformTree::kInvalidNodeId &&
       scroll_id != ScrollTree::kInvalidNodeId) {
     tree->property_trees()->scroll_tree.OnScrollOffsetAnimated(