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

Source/core/events/MouseEvent.h

 /*
  * Copyright (C) 2001 Peter Kelly (pmk@post.com)
  * Copyright (C) 2001 Tobias Anton (anton@stud.fbi.fh-darmstadt.de)
  * Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 59 matching lines @@
     void initMouseEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtrWillBeRawPtr<AbstractView>,
         int detail, int screenX, int screenY, int clientX, int clientY,
         bool ctrlKey, bool altKey, bool shiftKey, bool metaKey,
         unsigned short button, PassRefPtrWillBeRawPtr<EventTarget> relatedTarget);
 
     // WinIE uses 1,4,2 for left/middle/right but not for click (just for mousedown/up, maybe others),
     // but we will match the standard DOM.
     unsigned short button() const { return m_button; }
     bool buttonDown() const { return m_buttonDown; }
     EventTarget* relatedTarget() const { return m_relatedTarget.get(); }
-    EventTarget* relatedTarget(bool& isNull) const { isNull = !m_relatedTarget; return m_relatedTarget.get(); }
     void setRelatedTarget(PassRefPtrWillBeRawPtr<EventTarget> relatedTarget) { m_relatedTarget = relatedTarget; }
 
     Node* toElement() const;
     Node* fromElement() const;
 
     DataTransfer* dataTransfer() const { return isDragEvent() ? m_dataTransfer.get() : 0; }
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
     virtual bool isMouseEvent() const OVERRIDE;
@@ skipping 43 matching lines @@
     virtual bool dispatchEvent(EventDispatcher*) const OVERRIDE;
     bool isSyntheticMouseEvent() const { return m_mouseEventType == SyntheticMouseEvent; }
     MouseEventType m_mouseEventType;
 };
 
 DEFINE_EVENT_TYPE_CASTS(MouseEvent);
 
 } // namespace WebCore
 
 #endif // MouseEvent_h