Don't clear render surfaces unnecessarily.

cc/layers/layer_impl.h

 // 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.
 
 #ifndef CC_LAYERS_LAYER_IMPL_H_
 #define CC_LAYERS_LAYER_IMPL_H_
 
 #include <set>
 #include <string>
 
@@ skipping 257 matching lines @@
     return use_parent_backface_visibility_;
   }
 
   bool ShowDebugBorders() const;
 
   // These invalidate the host's render surface layer list.  The caller
   // is responsible for calling set_needs_update_draw_properties on the tree
   // so that its list can be recreated.
   void CreateRenderSurface();
   void ClearRenderSurface();
+  void ClearRenderSurfaceLayerList();
 
   DrawProperties<LayerImpl>& draw_properties() {
     return draw_properties_;
   }
   const DrawProperties<LayerImpl>& draw_properties() const {
     return draw_properties_;
   }
 
   // The following are shortcut accessors to get various information from
   // draw_properties_
@@ skipping 385 matching lines @@
   DrawProperties<LayerImpl> draw_properties_;
 
   scoped_refptr<base::debug::ConvertableToTraceFormat> debug_info_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_IMPL_H_