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

cc/trees/layer_tree_host_common.cc

Index: cc/trees/layer_tree_host_common.cc
diff --git a/cc/trees/layer_tree_host_common.cc b/cc/trees/layer_tree_host_common.cc
index 71d8e6be0a6626f00b14320fadb73cd0f29cbe5b..2787dbf5af3307f2d2dd925886b042c181175597 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -298,7 +298,8 @@ enum PropertyTreeOption {
 
 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;
@@ -320,7 +321,8 @@ static void ComputeInitialRenderSurfaceLayerList(
     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