Don't clear render surfaces unnecessarily.

cc/layers/render_surface.h

Index: cc/layers/render_surface.h
diff --git a/cc/layers/render_surface.h b/cc/layers/render_surface.h
index ace787a8d92e8d30b0b0eeb644634e03aa6a7704..4da8d23794268198bd5047b00ca789c09bdea881 100644
--- a/cc/layers/render_surface.h
+++ b/cc/layers/render_surface.h
@@ -101,7 +101,7 @@ class CC_EXPORT RenderSurface {
     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) {}
@@ -113,6 +113,8 @@ class CC_EXPORT RenderSurface {
     return nearest_occlusion_immune_ancestor_;
   }
 
+  void ClearLayerLists();
+
  private:
   friend class LayerIterator<Layer>;
 
@@ -136,7 +138,7 @@ class CC_EXPORT RenderSurface {
   // 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.