Implement checked attribute for menuitem.

Source/core/html/HTMLMenuItemElement.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef HTMLMenuItemElement_h
 #define HTMLMenuItemElement_h
 
 #include "core/html/HTMLElement.h"
 
 namespace blink {
 
 class HTMLMenuItemElement final : public HTMLElement {
     DEFINE_WRAPPERTYPEINFO();
 public:
     DECLARE_NODE_FACTORY(HTMLMenuItemElement);
 
 private:
     explicit HTMLMenuItemElement(Document&);
+    virtual void defaultEventHandler(Event*) override;
 };
 
 } // namespace blink
 
 #endif // HTMLMenuItemElement_h