cc : Determine if a layer is root by using the value in LayerTreeImpl

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index e89c84d102f23dc4cc59f406897b639a8e82fe6f..0420344247132d31c1a144f60545119b769a9d73 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -280,6 +280,10 @@ void LayerTreeImpl::SetRootLayer(scoped_ptr<LayerImpl> layer) {
   layer_tree_host_impl_->OnCanDrawStateChangedForTree();
 }
 
+bool LayerTreeImpl::IsRootLayer(const LayerImpl* layer) const {
+  return root_layer_ == layer;
+}
+
 LayerImpl* LayerTreeImpl::InnerViewportScrollLayer() const {
   return LayerById(inner_viewport_scroll_layer_id_);
 }