Dispatch change event for input type=checkbox when checked but value is null

Source/core/html/HTMLInputElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Samsung Electronics. 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
@@ skipping 73 matching lines @@
     // stepUp()/stepDown() for user-interaction.
     bool isSteppable() const;
 
     bool isTextButton() const;
 
     bool isRadioButton() const;
     bool isTextField() const;
     bool isSearchField() const;
     bool isInputTypeHidden() const;
     bool isPasswordField() const;
-    bool isCheckbox() const;
+    virtual bool isCheckbox() const OVERRIDE;
     bool isRangeControl() const;
 
     // FIXME: It's highly likely that any call site calling this function should instead
     // be using a different one. Many input elements behave like text fields, and in addition
     // any unknown input type is treated as text. Consider, for example, isTextField or
     // isTextField && !isPasswordField.
     bool isText() const;
 
     bool isEmailField() const;
     bool isFileUpload() const;
@@ skipping 294 matching lines @@
     RefPtr<InputTypeView> m_inputTypeView;
     // The ImageLoader must be owned by this element because the loader code assumes
     // that it lives as long as its owning element lives. If we move the loader into
     // the ImageInput object we may delete the loader while this element lives on.
     OwnPtr<HTMLImageLoader> m_imageLoader;
     OwnPtr<ListAttributeTargetObserver> m_listAttributeTargetObserver;
 };
 
 } //namespace
 #endif