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

Source/core/html/HTMLLabelElement.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.
  *
  * 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 16 matching lines @@
 #include "core/html/HTMLElement.h"
 #include "core/html/LabelableElement.h"
 
 namespace WebCore {
 
 class HTMLLabelElement FINAL : public HTMLElement {
 public:
     static PassRefPtr<HTMLLabelElement> create(Document&);
 
     LabelableElement* control();
-    HTMLFormElement* form() const;

[sof, 2013/12/10 17:26:05]

The removal of this method would have belonged better in the "fromOwner()" CL
(https://codereview.chromium.org/93723003/), but didn't end up there. Mea culpa.

No change in functionality.

 
     virtual bool willRespondToMouseClickEvents() OVERRIDE;
 
 private:
     explicit HTMLLabelElement(Document&);
     bool isInInteractiveContent(Node*) const;
 
     virtual bool rendererIsFocusable() const OVERRIDE;
     virtual bool isInteractiveContent() const OVERRIDE;
+    virtual bool isFormAssociatedElement() const OVERRIDE { return true; }
     virtual void accessKeyAction(bool sendMouseEvents);
 
     // Overridden to update the hover/active state of the corresponding control.
     virtual void setActive(bool = true) OVERRIDE;
     virtual void setHovered(bool = true) OVERRIDE;
 
     // Overridden to either click() or focus() the corresponding control.
     virtual void defaultEventHandler(Event*);
 
     virtual void focus(bool restorePreviousSelection, FocusDirection) OVERRIDE;
+
+    virtual HTMLFormElement* virtualForm() const;
 };
 
 inline bool isHTMLLabelElement(const Node* node)
 {
     return node->hasTagName(HTMLNames::labelTag);
 }
 
 inline bool isHTMLLabelElement(const Element* element)
 {
     return element->hasTagName(HTMLNames::labelTag);
 }
 
 DEFINE_NODE_TYPE_CASTS(HTMLLabelElement, hasTagName(HTMLNames::labelTag));
 
 } //namespace
 
 #endif