Don't clear render surfaces unnecessarily.

cc/layers/layer.h

 // 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.
 
 #ifndef CC_LAYERS_LAYER_H_
 #define CC_LAYERS_LAYER_H_
 
 #include <set>
 #include <string>
 
@@ skipping 354 matching lines @@
   virtual void OnOutputSurfaceCreated() {}
 
   virtual scoped_refptr<base::debug::ConvertableToTraceFormat> TakeDebugInfo();
 
   void SetLayerClient(LayerClient* client) { client_ = client; }
 
   virtual void PushPropertiesTo(LayerImpl* layer);
 
   void CreateRenderSurface();
   void ClearRenderSurface();
+  void ClearRenderSurfaceLayerList();
 
   // The contents scale converts from logical, non-page-scaled pixels to target
   // pixels. The contents scale is 1 for the root layer as it is already in
   // physical pixels. By default contents scale is forced to be 1 except for
   // subclasses of ContentsScalingLayer.
   float contents_scale_x() const { return draw_properties_.contents_scale_x; }
   float contents_scale_y() const { return draw_properties_.contents_scale_y; }
   gfx::Size content_bounds() const { return draw_properties_.content_bounds; }
 
   virtual void CalculateContentsScale(float ideal_contents_scale,
@@ skipping 239 matching lines @@
   DrawProperties<Layer> draw_properties_;
 
   PaintProperties paint_properties_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_H_