Drop [TypeChecking=Nullable]; deduce from attribute type instead

Source/core/events/MessageEvent.h

 /*
  * Copyright (C) 2007 Henry Mason (hmason@mac.com)
  * Copyright (C) 2003, 2004, 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 68 matching lines @@
     }
     static PassRefPtrWillBeRawPtr<MessageEvent> create(const AtomicString& type, const MessageEventInit& initializer, ExceptionState&);
     virtual ~MessageEvent();
 
     void initMessageEvent(const AtomicString& type, bool canBubble, bool cancelable, const String& origin, const String& lastEventId, LocalDOMWindow* source, PassOwnPtrWillBeRawPtr<MessagePortArray>);
     void initMessageEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtr<SerializedScriptValue> data, const String& origin, const String& lastEventId, LocalDOMWindow* source, PassOwnPtrWillBeRawPtr<MessagePortArray>);
 
     const String& origin() const { return m_origin; }
     const String& lastEventId() const { return m_lastEventId; }
     EventTarget* source() const { return m_source.get(); }
-    EventTarget* source(bool& isNull) const { isNull = !m_source; return m_source.get(); }
     MessagePortArray ports() const { return m_ports ? *m_ports : MessagePortArray(); }
     MessagePortChannelArray* channels() const { return m_channels ? m_channels.get() : 0; }
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
     enum DataType {
         DataTypeScriptValue,
         DataTypeSerializedScriptValue,
         DataTypeString,
         DataTypeBlob,
@@ skipping 37 matching lines @@
     // m_ports are the MessagePorts in an engtangled state, and m_channels are
     // the MessageChannels in a disentangled state. Only one of them can be
     // non-empty at a time. entangleMessagePorts() moves between the states.
     OwnPtrWillBeMember<MessagePortArray> m_ports;
     OwnPtr<MessagePortChannelArray> m_channels;
 };
 
 } // namespace WebCore
 
 #endif // MessageEvent_h