Use null Strings for default values of MessageEvent properties

Source/core/page/DOMWindow.cpp

Index: Source/core/page/DOMWindow.cpp
diff --git a/Source/core/page/DOMWindow.cpp b/Source/core/page/DOMWindow.cpp
index d77702d201afc518e81fdda952a8eb7394f5e8a3..a6956302f5356b122ff8136e443b387891f350c8 100644
--- a/Source/core/page/DOMWindow.cpp
+++ b/Source/core/page/DOMWindow.cpp
@@ -120,7 +120,7 @@ public:
     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(); }