Implement HTMLOptionElement.form as required by spec.

Source/core/html/HTMLOptionElement.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, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google 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
@@ skipping 46 matching lines @@
     void setLabel(const AtomicString&);
 
     bool ownElementDisabled() const { return m_disabled; }
 
     virtual bool isDisabledFormControl() const OVERRIDE;
 
     String textIndentedToRespectGroupLabel() const;
 
     void setSelectedState(bool);
 
+    HTMLFormElement* form() const;
+
 private:
     explicit HTMLOptionElement(Document&);
 
     virtual bool rendererIsFocusable() const OVERRIDE;
     virtual bool rendererIsNeeded(const RenderStyle&) { return false; }
     virtual void attach(const AttachContext& = AttachContext()) OVERRIDE;
     virtual void detach(const AttachContext& = AttachContext()) OVERRIDE;
 
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) OVERRIDE;
 
@@ skipping 14 matching lines @@
     bool m_disabled;
     bool m_isSelected;
     RefPtr<RenderStyle> m_style;
 };
 
 DEFINE_NODE_TYPE_CASTS(HTMLOptionElement, hasTagName(HTMLNames::optionTag));
 
 } // namespace WebCore
 
 #endif