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

cc/resources/picture_layer_tiling.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_RESOURCES_PICTURE_LAYER_TILING_H_
 #define CC_RESOURCES_PICTURE_LAYER_TILING_H_
 
 #include <set>
 #include <utility>
 #include <vector>
@@ skipping 26 matching lines @@
   // tiling) This might return null if the client cannot create such a tile.
   virtual scoped_refptr<Tile> CreateTile(
     PictureLayerTiling* tiling,
     const gfx::Rect& content_rect) = 0;
   virtual PicturePileImpl* GetPile() = 0;
   virtual gfx::Size CalculateTileSize(
     const gfx::Size& content_bounds) const = 0;
   // This invalidation region defines the area (if any, it can by null) that
   // tiles can not be shared between pending and active trees.
   virtual const Region* GetPendingInvalidation() = 0;
-  virtual const PictureLayerTiling* GetTwinTiling(
+  virtual const PictureLayerTiling* GetPendingOrActiveTwinTiling(
       const PictureLayerTiling* tiling) const = 0;
   virtual PictureLayerTiling* GetRecycledTwinTiling(
       const PictureLayerTiling* tiling) = 0;
   virtual size_t GetMaxTilesForInterestArea() const = 0;
   virtual float GetSkewportTargetTimeInSeconds() const = 0;
   virtual int GetSkewportExtrapolationLimitInContentPixels() const = 0;
   virtual WhichTree GetTree() const = 0;
   virtual bool RequiresHighResToDraw() const = 0;
 
  protected:
@@ skipping 321 matching lines @@
 
  private:
   DISALLOW_ASSIGN(PictureLayerTiling);
 
   RectExpansionCache expansion_cache_;
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_PICTURE_LAYER_TILING_H_