Don't clear render surfaces unnecessarily.

cc/layers/render_surface.cc

 // 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.
 
 #include "cc/layers/render_surface.h"
 
 #include "cc/base/math_util.h"
 #include "cc/layers/layer.h"
 #include "ui/gfx/transform.h"
 
@@ skipping 22 matching lines @@
 
 gfx::RectF RenderSurface::DrawableContentRect() const {
   gfx::RectF drawable_content_rect =
       MathUtil::MapClippedRect(draw_transform_, content_rect_);
   if (owning_layer_->has_replica())
     drawable_content_rect.Union(
         MathUtil::MapClippedRect(replica_draw_transform_, content_rect_));
   return drawable_content_rect;
 }
 
+void RenderSurface::ClearLayerLists() {
+  layer_list_.clear();
+}
+
 }  // namespace cc