[SyncFS] Use nullptr for scoped_ptr initialization

chrome/browser/sync_file_system/drive_backend/callback_tracker.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.h"
 
 #include <algorithm>
 
 namespace sync_file_system {
 namespace drive_backend {
@@ skipping 12 matching lines @@
        itr != helpers.end(); ++itr) {
     delete itr->first;
     itr->second.Run();
   }
 }
 
 scoped_ptr<internal::AbortHelper> CallbackTracker::PassAbortHelper(
     internal::AbortHelper* helper) {
   if (helpers_.erase(helper) == 1)
     return scoped_ptr<internal::AbortHelper>(helper);
-  return scoped_ptr<internal::AbortHelper>();
+  return nullptr;
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system