cc: Always keep the PictureLayerImpl::twin_layer_ pointer valid.

cc/test/fake_picture_layer_impl.h

 // Copyright 2013 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.
 
 #ifndef CC_TEST_FAKE_PICTURE_LAYER_IMPL_H_
 #define CC_TEST_FAKE_PICTURE_LAYER_IMPL_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "cc/layers/picture_layer_impl.h"
 
@@ skipping 62 matching lines @@
   float raster_page_scale() const { return raster_page_scale_; }
   void set_raster_page_scale(float scale) { raster_page_scale_ = scale; }
 
   float ideal_contents_scale() const { return ideal_contents_scale_; }
   float raster_contents_scale() const { return raster_contents_scale_; }
 
   PictureLayerTiling* HighResTiling() const;
   PictureLayerTiling* LowResTiling() const;
   size_t num_tilings() const { return tilings_->num_tilings(); }
 
-  PictureLayerImpl* twin_layer() { return twin_layer_; }
-  void set_twin_layer(PictureLayerImpl* twin) { twin_layer_ = twin; }
   PictureLayerTilingSet* tilings() { return tilings_.get(); }
   PicturePileImpl* pile() { return pile_.get(); }
+  void SetPile(scoped_refptr<PicturePileImpl> pile);
   size_t append_quads_count() { return append_quads_count_; }
 
   const Region& invalidation() const { return invalidation_; }
   void set_invalidation(const Region& region) { invalidation_ = region; }
 
   gfx::Rect visible_rect_for_tile_priority() {
     return visible_rect_for_tile_priority_;
   }
 
   void set_fixed_tile_size(const gfx::Size& size) { fixed_tile_size_ = size; }
@@ skipping 32 matching lines @@
   size_t append_quads_count_;
   size_t did_become_active_call_count_;
   bool has_valid_tile_priorities_;
   bool use_set_valid_tile_priorities_flag_;
   size_t release_resources_count_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TEST_FAKE_PICTURE_LAYER_IMPL_H_