[SyncFS] Use nullptr instead of NULL

chrome/browser/sync_file_system/drive_backend/local_to_remote_syncer.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/local_to_remote_syncer.h"
 
 #include <string>
 #include <vector>
 
 #include "base/callback.h"
@@ skipping 286 matching lines @@
     int64 latest_change_id = 0;
     if (!GetKnownChangeID(metadata_database(),
                           remote_file_tracker_->file_id(),
                           &latest_change_id) ||
         latest_change_id > remote_file_change_id_) {
       SyncCompleted(token.Pass(), SYNC_STATUS_RETRY);
       return;
     }
 
     if (!metadata_database()->FindTrackerByTrackerID(
-            remote_file_tracker_->tracker_id(), NULL)) {
+            remote_file_tracker_->tracker_id(), nullptr)) {
       SyncCompleted(token.Pass(), SYNC_STATUS_RETRY);
       return;
     }
   }
 
   continuation.Run(token.Pass());
 }
 
 void LocalToRemoteSyncer::SyncCompleted(scoped_ptr<SyncTaskToken> token,
                                         SyncStatusCode status) {
@@ skipping 427 matching lines @@
   set_used_network(true);
   return sync_context_->GetDriveUploader();
 }
 
 MetadataDatabase* LocalToRemoteSyncer::metadata_database() {
   return sync_context_->GetMetadataDatabase();
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system