Don't clear render surfaces unnecessarily.

cc/layers/render_surface.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_RENDER_SURFACE_H_
 #define CC_LAYERS_RENDER_SURFACE_H_
 
 #include <vector>
 
@@ skipping 83 matching lines @@
   // RenderSurface that is being drawn for the current frame. It could still be
   // drawn to as a target, but its output will not be a part of any other
   // surface.
   bool contributes_to_drawn_surface() const {
     return contributes_to_drawn_surface_;
   }
   void set_contributes_to_drawn_surface(bool contributes_to_drawn_surface) {
     contributes_to_drawn_surface_ = contributes_to_drawn_surface;
   }
 
-  RenderSurfaceLayerList& layer_list() { return layer_list_; }
+  LayerList& layer_list() { return layer_list_; }
   // A no-op since DelegatedRendererLayers on the main thread don't have any
   // RenderPasses so they can't contribute to a surface.
   void AddContributingDelegatedRenderPassLayer(Layer* layer) {}
 
   void SetNearestOcclusionImmuneAncestor(RenderSurface* surface) {
     nearest_occlusion_immune_ancestor_ = surface;
   }
   const RenderSurface* nearest_occlusion_immune_ancestor() const {
     return nearest_occlusion_immune_ancestor_;
   }
 
+  void ClearLayerLists();
+
  private:
   friend class LayerIterator<Layer>;
 
   Layer* owning_layer_;
 
   // Uses this surface's space.
   gfx::Rect content_rect_;
 
   float draw_opacity_;
   bool draw_opacity_is_animating_;
   gfx::Transform draw_transform_;
   gfx::Transform screen_space_transform_;
   gfx::Transform replica_draw_transform_;
   gfx::Transform replica_screen_space_transform_;
   bool target_surface_transforms_are_animating_;
   bool screen_space_transforms_are_animating_;
 
   bool is_clipped_;
   bool contributes_to_drawn_surface_;
 
   // Uses the space of the surface's target surface.
   gfx::Rect clip_rect_;
 
-  RenderSurfaceLayerList layer_list_;
+  LayerList layer_list_;
 
   // The nearest ancestor target surface that will contain the contents of this
   // surface, and that ignores outside occlusion. This can point to itself.
   RenderSurface* nearest_occlusion_immune_ancestor_;
 
   // For LayerIteratorActions
   int target_render_surface_layer_index_history_;
   int current_layer_index_history_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderSurface);
 };
 
 }  // namespace cc
 #endif  // CC_LAYERS_RENDER_SURFACE_H_