cc/blimp: Eliminate Layer updates for common cases.

cc/layers/layer.cc

 // Copyright 2010 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/layers/layer.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 402 matching lines @@
 void Layer::SetMaskLayer(Layer* mask_layer) {
   DCHECK(IsPropertyChangeAllowed());
   if (inputs_.mask_layer.get() == mask_layer)
     return;
   if (inputs_.mask_layer.get()) {
     DCHECK_EQ(this, inputs_.mask_layer->parent());
     inputs_.mask_layer->RemoveFromParent();
   }
   inputs_.mask_layer = mask_layer;
   if (inputs_.mask_layer.get()) {
+    // The mask layer should not have any children.
+    DCHECK(inputs_.mask_layer->children().empty());
+
     inputs_.mask_layer->RemoveFromParent();
     DCHECK(!inputs_.mask_layer->parent());
     inputs_.mask_layer->SetParent(this);
     inputs_.mask_layer->SetIsMask(true);
   }
   SetSubtreePropertyChanged();
   SetNeedsFullTreeSync();
 }
 
 void Layer::SetFilters(const FilterOperations& filters) {
@@ skipping 1456 matching lines @@
 
 LayerTree* Layer::GetLayerTree() const {
   return layer_tree_;
 }
 
 void Layer::SetLayerIdForTesting(int id) {
   inputs_.layer_id = id;
 }
 
 }  // namespace cc