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 25 matching lines @@
 class ValidationMessage;
 class ValidityState;
 
 // HTMLFormControlElement is the default implementation of FormAssociatedElement,
 // and form-associated element implementations should use HTMLFormControlElement
 // unless there is a special reason.
 class HTMLFormControlElement : public LabelableElement, public FormAssociatedElement {
 public:
     virtual ~HTMLFormControlElement();
 
-    HTMLFormElement* form() const { return FormAssociatedElement::form(); }
-
     String formEnctype() const;
     void setFormEnctype(const AtomicString&);
     String formMethod() const;
     void setFormMethod(const AtomicString&);
     bool formNoValidate() const;
 
     void ancestorDisabledStateWasChanged();
 
     void reset();
 
@@ skipping 80 matching lines @@
     void setNeedsWillValidateCheck();
     virtual bool recalcWillValidate() const;
 
     virtual void resetImpl() { }
 
 private:
     virtual void refFormAssociatedElement() { ref(); }
     virtual void derefFormAssociatedElement() { deref(); }
 
     virtual bool isFormControlElement() const { return true; }
+    virtual bool isFormAssociatedElement() const OVERRIDE { 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;
@@ skipping 50 matching lines @@
 
 inline const HTMLFormControlElement& toHTMLFormControlElement(const FormAssociatedElement& control)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(control.isFormControlElement());
     return static_cast<const HTMLFormControlElement&>(control);
 }
 
 } // namespace
 
 #endif