Don't clear render surfaces unnecessarily.

cc/layers/layer_iterator_unittest.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_iterator.h"
 
 #include <vector>
 
 #include "cc/layers/layer.h"
 #include "cc/test/fake_layer_tree_host.h"
@@ skipping 50 matching lines @@
     RenderSurface* render_surface = render_surface_layer->render_surface();
 
     render_surface_layer->count_representing_target_surface_ = -1;
     render_surface_layer->count_representing_contributing_surface_ = -1;
     render_surface_layer->count_representing_itself_ = -1;
 
     for (unsigned layer_index = 0;
          layer_index < render_surface->layer_list().size();
          ++layer_index) {
       TestLayer* layer = static_cast<TestLayer*>(
-          render_surface->layer_list().at(layer_index));
+          render_surface->layer_list().at(layer_index).get());
 
       layer->count_representing_target_surface_ = -1;
       layer->count_representing_contributing_surface_ = -1;
       layer->count_representing_itself_ = -1;
     }
   }
 }
 
 void IterateFrontToBack(
     RenderSurfaceLayerList* render_surface_layer_list) {
@@ skipping 126 matching lines @@
   EXPECT_COUNT(root21, -1, -1, 9);
   EXPECT_COUNT(root22, 7, 8, 6);
   EXPECT_COUNT(root221, -1, -1, 5);
   EXPECT_COUNT(root23, 3, 4, 2);
   EXPECT_COUNT(root231, -1, -1, 1);
   EXPECT_COUNT(root3, -1, -1, 0);
 }
 
 }  // namespace
 }  // namespace cc