cc: Remove use of PassAs() and constructor-casting with scoped_ptr.

cc/trees/tree_synchronizer.cc

 // Copyright 2011 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/trees/tree_synchronizer.h"
 
 #include <set>
 
 #include "base/containers/hash_tables.h"
 #include "base/containers/scoped_ptr_hash_map.h"
@@ skipping 87 matching lines @@
   return layer_impl.Pass();
 }
 
 template <typename LayerType>
 scoped_ptr<LayerImpl> SynchronizeTreesRecursiveInternal(
     RawPtrLayerImplMap* new_layers,
     ScopedPtrLayerImplMap* old_layers,
     LayerType* layer,
     LayerTreeImpl* tree_impl) {
   if (!layer)
-    return scoped_ptr<LayerImpl>();
+    return nullptr;
 
   scoped_ptr<LayerImpl> layer_impl =
       ReuseOrCreateLayerImpl(new_layers, old_layers, layer, tree_impl);
 
   layer_impl->ClearChildList();
   for (size_t i = 0; i < layer->children().size(); ++i) {
     layer_impl->AddChild(SynchronizeTreesRecursiveInternal(
         new_layers, old_layers, layer->child_at(i), tree_impl));
   }
 
@@ skipping 183 matching lines @@
 #endif
 }
 
 void TreeSynchronizer::PushProperties(LayerImpl* layer, LayerImpl* layer_impl) {
   size_t num_dependents_need_push_properties = 0;
   PushPropertiesInternal(
       layer, layer_impl, &num_dependents_need_push_properties);
 }
 
 }  // namespace cc