[SyncFS] Use nullptr for scoped_ptr initialization

chrome/browser/sync_file_system/drive_backend/callback_tracker_internal.cc

 // Copyright 2014 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 "chrome/browser/sync_file_system/drive_backend/callback_tracker_internal.h"
 
 #include "chrome/browser/sync_file_system/drive_backend/callback_tracker.h"
 
 namespace sync_file_system {
 namespace drive_backend {
 namespace internal {
 
 AbortHelper::AbortHelper(CallbackTracker* tracker)
     : tracker_(tracker), weak_ptr_factory_(this) {
 }
 
 AbortHelper::~AbortHelper() {}
 
 base::WeakPtr<AbortHelper> AbortHelper::AsWeakPtr() {
   return weak_ptr_factory_.GetWeakPtr();
 }
 
 // static
 scoped_ptr<AbortHelper> AbortHelper::TakeOwnership(
     const base::WeakPtr<AbortHelper>& abort_helper) {
   if (!abort_helper)
-    return scoped_ptr<AbortHelper>();
+    return nullptr;
   scoped_ptr<AbortHelper> result =
       abort_helper->tracker_->PassAbortHelper(abort_helper.get());
   abort_helper->weak_ptr_factory_.InvalidateWeakPtrs();
   return result.Pass();
 }
 
 }  // namespace internal
 }  // namespace drive_backend
 }  // namespace sync_file_system