Don't clear render surfaces unnecessarily.

cc/layers/layer.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/layer.h"
 
 #include <algorithm>
 
 #include "base/debug/trace_event.h"
 #include "base/location.h"
@@ skipping 1042 matching lines @@
 void Layer::CreateRenderSurface() {
   DCHECK(!draw_properties_.render_surface);
   draw_properties_.render_surface = make_scoped_ptr(new RenderSurface(this));
   draw_properties_.render_target = this;
 }
 
 void Layer::ClearRenderSurface() {
   draw_properties_.render_surface.reset();
 }
 
+void Layer::ClearRenderSurfaceLayerList() {
+  if (draw_properties_.render_surface)
+    draw_properties_.render_surface->layer_list().clear();
+}
+
 gfx::Vector2dF Layer::ScrollOffsetForAnimation() const {
   return TotalScrollOffset();
 }
 
 // On<Property>Animated is called due to an ongoing accelerated animation.
 // Since this animation is also being run on the compositor thread, there
 // is no need to request a commit to push this value over, so the value is
 // set directly rather than by calling Set<Property>.
 void Layer::OnFilterAnimated(const FilterOperations& filters) {
   filters_ = filters;
@@ skipping 106 matching lines @@
   if (clip_parent_)
     clip_parent_->RemoveClipChild(this);
 
   clip_parent_ = NULL;
 }
 
 void Layer::RunMicroBenchmark(MicroBenchmark* benchmark) {
   benchmark->RunOnLayer(this);
 }
 }  // namespace cc