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

cc/trees/layer_tree_impl.h

 // 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.
 
 #ifndef CC_TREES_LAYER_TREE_IMPL_H_
 #define CC_TREES_LAYER_TREE_IMPL_H_
 
 #include <list>
 #include <set>
 #include <string>
@@ skipping 59 matching lines @@
   TileManager* tile_manager() const;
   FrameRateCounter* frame_rate_counter() const;
   PaintTimeCounter* paint_time_counter() const;
   MemoryHistory* memory_history() const;
   gfx::Size device_viewport_size() const;
   bool IsActiveTree() const;
   bool IsPendingTree() const;
   bool IsRecycleTree() const;
   LayerImpl* FindActiveTreeLayerById(int id);
   LayerImpl* FindPendingTreeLayerById(int id);
-  LayerImpl* FindRecycleTreeLayerById(int id);
   bool PinchGestureActive() const;
   BeginFrameArgs CurrentBeginFrameArgs() const;
   base::TimeDelta begin_impl_frame_interval() const;
   void SetNeedsCommit();
   gfx::Rect DeviceViewport() const;
   gfx::Size DrawViewportSize() const;
   const gfx::Rect ViewportRectForTilePriority() const;
   scoped_ptr<ScrollbarAnimationController> CreateScrollbarAnimationController(
       LayerImpl* scrolling_layer);
   void DidAnimateScrollOffset();
@@ skipping 299 matching lines @@
 
   scoped_ptr<PageScaleAnimation> page_scale_animation_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerTreeImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_IMPL_H_