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

cc/test/fake_picture_layer_impl.cc

 // 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.
 
 #include "cc/test/fake_picture_layer_impl.h"
 
 #include <vector>
 #include "cc/resources/tile.h"
 #include "cc/trees/layer_tree_impl.h"
 
@@ skipping 79 matching lines @@
     PictureLayerTiling* tiling = tilings_->tiling_at(i);
     if (tiling->resolution() == LOW_RESOLUTION) {
       // There should be only one low res tiling.
       CHECK(!result);
       result = tiling;
     }
   }
   return result;
 }
 
+void FakePictureLayerImpl::SetPile(scoped_refptr<PicturePileImpl> pile) {
+  pile_.swap(pile);
+  if (tilings()) {
+    for (size_t i = 0; i < num_tilings(); ++i) {
+      tilings()->tiling_at(i)->UpdateTilesToCurrentPile(Region(),
+                                                        pile_->tiling_size());
+    }
+  }
+}
+
 void FakePictureLayerImpl::SetAllTilesVisible() {
   WhichTree tree =
       layer_tree_impl()->IsActiveTree() ? ACTIVE_TREE : PENDING_TREE;
 
   for (size_t tiling_idx = 0; tiling_idx < tilings_->num_tilings();
        ++tiling_idx) {
     PictureLayerTiling* tiling = tilings_->tiling_at(tiling_idx);
     std::vector<Tile*> tiles = tiling->AllTilesForTesting();
     for (size_t tile_idx = 0; tile_idx < tiles.size(); ++tile_idx) {
       Tile* tile = tiles[tile_idx];
@@ skipping 69 matching lines @@
              ? has_valid_tile_priorities_
              : PictureLayerImpl::HasValidTilePriorities();
 }
 
 void FakePictureLayerImpl::ReleaseResources() {
   PictureLayerImpl::ReleaseResources();
   ++release_resources_count_;
 }
 
 }  // namespace cc