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

cc/resources/tile_manager_unittest.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/resources/eviction_tile_priority_queue.h"
 #include "cc/resources/raster_tile_priority_queue.h"
 #include "cc/resources/tile.h"
 #include "cc/resources/tile_priority.h"
 #include "cc/test/fake_impl_proxy.h"
 #include "cc/test/fake_layer_tree_host_impl.h"
@@ skipping 82 matching lines @@
   void SetupTrees(scoped_refptr<PicturePileImpl> pending_pile,
                   scoped_refptr<PicturePileImpl> active_pile) {
     SetupPendingTree(active_pile);
     ActivateTree();
     SetupPendingTree(pending_pile);
   }
 
   void SetupPendingTree(scoped_refptr<PicturePileImpl> pile) {
     host_impl_.CreatePendingTree();
     LayerTreeImpl* pending_tree = host_impl_.pending_tree();
-    // Clear recycled tree.
-    pending_tree->DetachLayerTree();
 
-    scoped_ptr<FakePictureLayerImpl> pending_layer =
-        FakePictureLayerImpl::CreateWithPile(pending_tree, id_, pile);
-    pending_layer->SetDrawsContent(true);
+    // Steal from the recycled tree.
+    scoped_ptr<LayerImpl> old_pending_root = pending_tree->DetachLayerTree();
+    DCHECK_IMPLIES(old_pending_root, old_pending_root->id() == id_);
+
+    scoped_ptr<FakePictureLayerImpl> pending_layer;
+    if (old_pending_root) {
+      pending_layer.reset(
+          static_cast<FakePictureLayerImpl*>(old_pending_root.release()));
+      pending_layer->SetPile(pile);
+    } else {
+      pending_layer =
+          FakePictureLayerImpl::CreateWithPile(pending_tree, id_, pile);
+      pending_layer->SetDrawsContent(true);
+    }
+    // The bounds() just mirror the pile size.
+    pending_layer->SetBounds(pending_layer->pile()->tiling_size());
     pending_tree->SetRootLayer(pending_layer.Pass());
 
     pending_layer_ = static_cast<FakePictureLayerImpl*>(
         host_impl_.pending_tree()->LayerById(id_));
     pending_layer_->DoPostCommitInitializationIfNeeded();
   }
 
   void CreateHighLowResAndSetAllTilesVisible() {
     // Active layer must get updated first so pending layer can share from it.
     active_layer_->CreateDefaultTilingsAndTiles();
@@ skipping 561 matching lines @@
     all_tiles.insert(queue.Top());
     ++tile_count;
     queue.Pop();
   }
   EXPECT_EQ(tile_count, all_tiles.size());
   EXPECT_EQ(17u, tile_count);
 }
 
 }  // namespace
 }  // namespace cc