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

cc/layers/picture_layer_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_LAYERS_PICTURE_LAYER_IMPL_H_
 #define CC_LAYERS_PICTURE_LAYER_IMPL_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 104 matching lines @@
   void DidBeginTracing() override;
   void ReleaseResources() override;
   skia::RefPtr<SkPicture> GetPicture() override;
 
   // PictureLayerTilingClient overrides.
   scoped_refptr<Tile> CreateTile(PictureLayerTiling* tiling,
                                  const gfx::Rect& content_rect) override;
   PicturePileImpl* GetPile() override;
   gfx::Size CalculateTileSize(const gfx::Size& content_bounds) const override;
   const Region* GetPendingInvalidation() override;
-  const PictureLayerTiling* GetTwinTiling(
+  const PictureLayerTiling* GetPendingOrActiveTwinTiling(
       const PictureLayerTiling* tiling) const override;
   PictureLayerTiling* GetRecycledTwinTiling(
       const PictureLayerTiling* tiling) override;
   size_t GetMaxTilesForInterestArea() const override;
   float GetSkewportTargetTimeInSeconds() const override;
   int GetSkewportExtrapolationLimitInContentPixels() const override;
   WhichTree GetTree() const override;
   bool RequiresHighResToDraw() const override;
 
   // PushPropertiesTo active tree => pending tree.
   void SyncTiling(const PictureLayerTiling* tiling);
 
   // Mask-related functions.
   void GetContentsResourceId(ResourceProvider::ResourceId* resource_id,
                              gfx::Size* resource_size) const override;
 
   size_t GPUMemoryUsageInBytes() const override;
 
   void RunMicroBenchmark(MicroBenchmarkImpl* benchmark) override;
 
   // Functions used by tile manager.
-  PictureLayerImpl* GetTwinLayer() { return twin_layer_; }
+  PictureLayerImpl* GetPendingOrActiveTwinLayer() const;
   bool IsOnActiveOrPendingTree() const;
   // Virtual for testing.
   virtual bool HasValidTilePriorities() const;
   bool AllTilesRequiredForActivationAreReadyToDraw() const;
 
  protected:
   friend class LayerRasterTileIterator;
 
   PictureLayerImpl(LayerTreeImpl* tree_impl, int id);
   PictureLayerTiling* AddTiling(float contents_scale);
   void RemoveTiling(float contents_scale);
   void RemoveAllTilings();
   void SyncFromActiveLayer(const PictureLayerImpl* other);
   void AddTilingsForRasterScale();
   void UpdateTilePriorities(const Occlusion& occlusion_in_content_space);
   virtual bool ShouldAdjustRasterScale() const;
   virtual void RecalculateRasterScales();
   void CleanUpTilingsOnActiveLayer(
       std::vector<PictureLayerTiling*> used_tilings);
   float MinimumContentsScale() const;
   float SnappedContentsScale(float new_contents_scale);
   void ResetRasterScale();
   gfx::Rect GetViewportForTilePriorityInContentSpace() const;
-  PictureLayerImpl* GetRecycledTwinLayer();
+  PictureLayerImpl* GetRecycledTwinLayer() const;
 
   void DoPostCommitInitializationIfNeeded() {
     if (needs_post_commit_initialization_)
       DoPostCommitInitialization();
   }
   void DoPostCommitInitialization();
 
   bool CanHaveTilings() const;
   bool CanHaveTilingWithScale(float contents_scale) const;
   void SanityCheckTilingState() const;
@@ skipping 40 matching lines @@
   // frame that has a valid viewport for prioritizing tiles.
   gfx::Rect visible_rect_for_tile_priority_;
 
   friend class PictureLayer;
   DISALLOW_COPY_AND_ASSIGN(PictureLayerImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_PICTURE_LAYER_IMPL_H_