Use null Strings for default values of MessageEvent properties

Source/core/dom/MessageEvent.cpp

 /*
  * Copyright (C) 2007 Henry Mason (hmason@mac.com)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008 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
@@ skipping 74 matching lines @@
     if (m_dataAsSerializedScriptValue)
         m_dataAsSerializedScriptValue->registerMemoryAllocatedWithCurrentScriptContext();
     ASSERT(isValidSource(m_source.get()));
 }
 
 MessageEvent::MessageEvent(const String& data, const String& origin)
     : Event(eventNames().messageEvent, false, false)
     , m_dataType(DataTypeString)
     , m_dataAsString(data)
     , m_origin(origin)
-    , m_lastEventId("")
 {
     ScriptWrappable::init(this);
 }
 
 MessageEvent::MessageEvent(PassRefPtr<Blob> data, const String& origin)
     : Event(eventNames().messageEvent, false, false)
     , m_dataType(DataTypeBlob)
     , m_dataAsBlob(data)
     , m_origin(origin)
-    , m_lastEventId("")
 {
     ScriptWrappable::init(this);
 }
 
 MessageEvent::MessageEvent(PassRefPtr<ArrayBuffer> data, const String& origin)
     : Event(eventNames().messageEvent, false, false)
     , m_dataType(DataTypeArrayBuffer)
     , m_dataAsArrayBuffer(data)
     , m_origin(origin)
-    , m_lastEventId("")
 {
     ScriptWrappable::init(this);
 }
 
 MessageEvent::~MessageEvent()
 {
 }
 
 void MessageEvent::initMessageEvent(const AtomicString& type, bool canBubble, bool cancelable, const String& origin, const String& lastEventId, DOMWindow* source, PassOwnPtr<MessagePortArray> ports)
 {
@@ skipping 26 matching lines @@
     if (m_dataAsSerializedScriptValue)
         m_dataAsSerializedScriptValue->registerMemoryAllocatedWithCurrentScriptContext();
 }
 
 const AtomicString& MessageEvent::interfaceName() const
 {
     return eventNames().interfaceForMessageEvent;
 }
 
 } // namespace WebCore