[SyncFS] Use nullptr for scoped_ptr initialization

chrome/browser/sync_file_system/drive_backend/metadata_database.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 "chrome/browser/sync_file_system/drive_backend/metadata_database.h"
 
 #include <algorithm>
 #include <stack>
 
 #include "base/bind.h"
@@ skipping 172 matching lines @@
   app_root_tracker->set_tracker_kind(TRACKER_KIND_REGULAR);
   app_root_tracker->set_dirty(false);
   app_root_tracker->set_active(false);
   app_root_tracker->set_needs_folder_listing(false);
   *app_root_tracker->mutable_synced_details() = app_root_metadata.details();
   return app_root_tracker.Pass();
 }
 
 scoped_ptr<FileTracker> CloneFileTracker(const FileTracker* obj) {
   if (!obj)
-    return scoped_ptr<FileTracker>();
+    return nullptr;
   return scoped_ptr<FileTracker>(new FileTracker(*obj));
 }
 
 // Returns true if |db| has no content.
 bool IsDatabaseEmpty(LevelDBWrapper* db) {
   DCHECK(db);
   scoped_ptr<LevelDBWrapper::Iterator> itr(db->NewIterator());
   itr->SeekToFirst();
   return !itr->Valid();
 }
@@ skipping 1587 matching lines @@
       return false;
 
   if (!parents.empty())
     return false;
 
   return true;
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system