Don't clear render surfaces unnecessarily.

cc/layers/layer_lists.h

 // Copyright 2013 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.
 
 #ifndef CC_LAYERS_LAYER_LISTS_H_
 #define CC_LAYERS_LAYER_LISTS_H_
 
 #include <vector>
 
 #include "base/memory/ref_counted.h"
@@ skipping 20 matching lines @@
   Layer* back();
   size_t size() const;
   bool empty() const { return size() == 0u; }
   scoped_refptr<Layer>& operator[](size_t i);
   const scoped_refptr<Layer>& operator[](size_t i) const;
   LayerList::iterator begin();
   LayerList::iterator end();
   LayerList::const_iterator begin() const;
   LayerList::const_iterator end() const;
   void clear();
+  LayerList& AsLayerList() { return list_; }
+  const LayerList& AsLayerList() const { return list_; }
 
  private:
   LayerList list_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderSurfaceLayerList);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_LISTS_H_