cc: Stop creating unused 0 property tree nodes other than transform

cc/trees/layer_tree_host_common.cc

 // Copyright 2011 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/trees/layer_tree_host_common.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 280 matching lines @@
   return jitter;
 }
 
 enum PropertyTreeOption {
   BUILD_PROPERTY_TREES_IF_NEEDED,
   DONT_BUILD_PROPERTY_TREES
 };
 
 static void ComputeLayerScrollsDrawnDescendants(LayerTreeImpl* layer_tree_impl,
                                                 ScrollTree* scroll_tree) {
-  for (int i = static_cast<int>(scroll_tree->size()) - 1; i > 0; --i) {
+  for (int i = static_cast<int>(scroll_tree->size()) - 1;
+       i > ScrollTree::kRootNodeId; --i) {
     ScrollNode* node = scroll_tree->Node(i);
     scroll_tree->parent(node)->data.num_drawn_descendants +=
         node->data.num_drawn_descendants;
   }
   for (LayerImpl* layer : *layer_tree_impl) {
     bool scrolls_drawn_descendant = false;
     if (layer->scrollable()) {
       ScrollNode* node = scroll_tree->Node(layer->scroll_tree_index());
       if (node->data.num_drawn_descendants > 0)
         scrolls_drawn_descendant = true;
     }
     layer->set_scrolls_drawn_descendant(scrolls_drawn_descendant);
   }
 }
 
 static void ComputeInitialRenderSurfaceLayerList(
     LayerTreeImpl* layer_tree_impl,
     PropertyTrees* property_trees,
     LayerImplList* render_surface_layer_list,
     bool can_render_to_separate_surface) {
   ScrollTree* scroll_tree = &property_trees->scroll_tree;
-  for (int i = 0; i < static_cast<int>(scroll_tree->size()); ++i)
+  for (int i = ScrollTree::kRootNodeId;
+       i < static_cast<int>(scroll_tree->size()); ++i)
     scroll_tree->Node(i)->data.num_drawn_descendants = 0;
 
   // Add all non-skipped surfaces to the initial render surface layer list. Add
   // all non-skipped layers to the layer list of their target surface, and
   // add their content rect to their target surface's accumulated content rect.
   for (LayerImpl* layer : *layer_tree_impl) {
     if (layer->render_surface()) {
       layer->ClearRenderSurfaceLayerList();
       ClearMaskLayersAreDrawnRenderSurfaceLayerListMembers(
           layer->render_surface());
@@ skipping 350 matching lines @@
 
 PropertyTrees* GetPropertyTrees(Layer* layer) {
   return layer->layer_tree_host()->property_trees();
 }
 
 PropertyTrees* GetPropertyTrees(LayerImpl* layer) {
   return layer->layer_tree_impl()->property_trees();
 }
 
 }  // namespace cc