Use null Strings for default values of MessageEvent properties

Source/core/page/DOMWindow.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  *
  * 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
@@ skipping 101 matching lines @@
         , m_source(source)
         , m_channels(channels)
         , m_targetOrigin(targetOrigin)
         , m_stackTrace(stackTrace)
     {
     }
 
     PassRefPtr<MessageEvent> event(ScriptExecutionContext* context)
     {
         OwnPtr<MessagePortArray> messagePorts = MessagePort::entanglePorts(*context, m_channels.release());
-        return MessageEvent::create(messagePorts.release(), m_message, m_origin, "", m_source);
+        return MessageEvent::create(messagePorts.release(), m_message, m_origin, String(), m_source);
     }
     SecurityOrigin* targetOrigin() const { return m_targetOrigin.get(); }
     ScriptCallStack* stackTrace() const { return m_stackTrace.get(); }
 
 private:
     virtual void fired()
     {
         m_window->postMessageTimerFired(adoptPtr(this));
         // This object is deleted now.
     }
@@ skipping 1627 matching lines @@
     return static_cast<DOMWindowLifecycleNotifier*>(LifecycleContext::lifecycleNotifier());
 }
 
 PassOwnPtr<LifecycleNotifier> DOMWindow::createLifecycleNotifier()
 {
     return DOMWindowLifecycleNotifier::create(this);
 }
 
 
 } // namespace WebCore