Don't clear render surfaces unnecessarily.

cc/layers/layer_impl.cc

 // Copyright 2012 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_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "base/json/json_reader.h"
 #include "base/strings/stringprintf.h"
 #include "cc/animation/animation_registrar.h"
@@ skipping 222 matching lines @@
   DCHECK(!draw_properties_.render_surface);
   draw_properties_.render_surface =
       make_scoped_ptr(new RenderSurfaceImpl(this));
   draw_properties_.render_target = this;
 }
 
 void LayerImpl::ClearRenderSurface() {
   draw_properties_.render_surface.reset();
 }
 
+void LayerImpl::ClearRenderSurfaceLayerList() {
+  if (draw_properties_.render_surface)
+    draw_properties_.render_surface->layer_list().clear();
+}
+
 scoped_ptr<SharedQuadState> LayerImpl::CreateSharedQuadState() const {
   scoped_ptr<SharedQuadState> state = SharedQuadState::Create();
   state->SetAll(draw_properties_.target_space_transform,
                 draw_properties_.content_bounds,
                 draw_properties_.visible_content_rect,
                 draw_properties_.clip_rect,
                 draw_properties_.is_clipped,
                 draw_properties_.opacity,
                 blend_mode_);
   return state.Pass();
@@ skipping 1273 matching lines @@
 scoped_ptr<base::Value> LayerImpl::AsValue() const {
   scoped_ptr<base::DictionaryValue> state(new base::DictionaryValue());
   AsValueInto(state.get());
   return state.PassAs<base::Value>();
 }
 
 void LayerImpl::RunMicroBenchmark(MicroBenchmarkImpl* benchmark) {
   benchmark->RunOnLayer(this);
 }
 }  // namespace cc