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

cc/trees/layer_tree_impl.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_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 262 matching lines @@
 
 void LayerTreeImpl::SetRootLayer(scoped_ptr<LayerImpl> layer) {
   if (root_layer_ && layer.get() != root_layer_)
     RemoveLayer(root_layer_->id());
   root_layer_ = layer.get();
   if (layer)
     AddLayer(std::move(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_);
 }
 
 LayerImpl* LayerTreeImpl::OuterViewportScrollLayer() const {
   return LayerById(outer_viewport_scroll_layer_id_);
 }
 
 gfx::ScrollOffset LayerTreeImpl::TotalScrollOffset() const {
   gfx::ScrollOffset offset;
@@ skipping 1809 matching lines @@
 }
 
 void LayerTreeImpl::ResetAllChangeTracking(PropertyTrees::ResetFlags flag) {
   layers_that_should_push_properties_.clear();
   for (auto* layer : *this)
     layer->ResetChangeTracking();
   property_trees_.ResetAllChangeTracking(flag);
 }
 
 }  // namespace cc