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

cc/test/fake_picture_layer_impl.h

Index: cc/test/fake_picture_layer_impl.h
diff --git a/cc/test/fake_picture_layer_impl.h b/cc/test/fake_picture_layer_impl.h
index ef554b3a7bc02269be5bcc099aa5ea6eac93ffed..a3c69c56fb6c1f0acae44fbfdf894f5773913a0f 100644
--- a/cc/test/fake_picture_layer_impl.h
+++ b/cc/test/fake_picture_layer_impl.h
@@ -80,10 +80,9 @@ class FakePictureLayerImpl : public PictureLayerImpl {
   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_; }