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 0263d5537c286c66985dc5c18281a7356d6deafc..cf7e1f56765e869692602d91d6ba084c1dbd3d14 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -302,7 +302,8 @@ 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)->num_drawn_descendants +=
         node->num_drawn_descendants;
@@ -324,7 +325,8 @@ 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)->num_drawn_descendants = 0;
 
   // Add all non-skipped surfaces to the initial render surface layer list. Add