Only 'form associated' elements have a potential form owner.

Source/core/html/HTMLFormControlElement.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, 2008, 2009, 2010 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 44 matching lines @@
 
     virtual bool formControlValueMatchesRenderer() const { return m_valueMatchesRenderer; }
     virtual void setFormControlValueMatchesRenderer(bool b) { m_valueMatchesRenderer = b; }
 
     virtual bool wasChangedSinceLastFormControlChangeEvent() const;
     virtual void setChangedSinceLastFormControlChangeEvent(bool);
 
     virtual void dispatchFormControlChangeEvent();
     virtual void dispatchFormControlInputEvent();
 
+    virtual HTMLFormElement* formOwner() const OVERRIDE;
+
     virtual bool isDisabledFormControl() const OVERRIDE;
 
     virtual bool isEnumeratable() const { return false; }
 
     bool isRequired() const;
 
     const AtomicString& type() const { return formControlType(); }
 
     virtual const AtomicString& formControlType() const OVERRIDE = 0;
 
@@ skipping 65 matching lines @@
 
 private:
     virtual void refFormAssociatedElement() { ref(); }
     virtual void derefFormAssociatedElement() { deref(); }
 
     virtual bool isFormControlElement() const { return true; }
     virtual bool alwaysCreateUserAgentShadowRoot() const OVERRIDE { return true; }
 
     virtual short tabIndex() const;
 
-    virtual HTMLFormElement* virtualForm() const;
     virtual bool isDefaultButtonForForm() const;
     virtual bool isValidFormControlElement();
     void updateAncestorDisabledState() const;
 
     OwnPtr<ValidationMessage> m_validationMessage;
     bool m_disabled : 1;
     bool m_isAutofilled : 1;
     bool m_isReadOnly : 1;
     bool m_isRequired : 1;
     bool m_valueMatchesRenderer : 1;
@@ skipping 45 matching lines @@
 
 inline const HTMLFormControlElement& toHTMLFormControlElement(const FormAssociatedElement& control)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(control.isFormControlElement());
     return static_cast<const HTMLFormControlElement&>(control);
 }
 
 } // namespace
 
 #endif