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

Source/core/html/HTMLLabelElement.h

Index: Source/core/html/HTMLLabelElement.h
diff --git a/Source/core/html/HTMLLabelElement.h b/Source/core/html/HTMLLabelElement.h
index 275b0777e5b8a9020e894cc886075fa6e7c34986..f1fc93de4146d1a26c93bb3c61da527c57615cdd 100644
--- a/Source/core/html/HTMLLabelElement.h
+++ b/Source/core/html/HTMLLabelElement.h
@@ -34,7 +34,6 @@ 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;
 
@@ -44,6 +43,7 @@ private:
 
     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.
@@ -54,6 +54,8 @@ private:
     virtual void defaultEventHandler(Event*);
 
     virtual void focus(bool restorePreviousSelection, FocusDirection) OVERRIDE;
+
+    virtual HTMLFormElement* virtualForm() const;
 };
 
 inline bool isHTMLLabelElement(const Node* node)