Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/modules/webdatabase/DatabaseServer.cpp

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 56 matching lines @@
     RefPtr<DatabaseBackendBase> database;
     switch (type) {
     case DatabaseType::Async:
         database = adoptRef(new Database(backendContext, name, expectedVersion, displayName, estimatedSize));
         break;
     case DatabaseType::Sync:
         database = adoptRef(new DatabaseSync(backendContext, name, expectedVersion, displayName, estimatedSize));
     }
 
     if (!database->openAndVerifyVersion(setVersionInNewDatabase, error, errorMessage))
-        return 0;
+        return nullptr;
     return database.release();
 }
 
 } // namespace WebCore