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

Source/core/html/HTMLFormControlElement.h

Index: Source/core/html/HTMLFormControlElement.h
diff --git a/Source/core/html/HTMLFormControlElement.h b/Source/core/html/HTMLFormControlElement.h
index 391dae36441335f283b1c1b035e5ecab34e9b370..9438b6a775d10539a569e992b89143de4201421f 100644
--- a/Source/core/html/HTMLFormControlElement.h
+++ b/Source/core/html/HTMLFormControlElement.h
@@ -43,8 +43,6 @@ class HTMLFormControlElement : public LabelableElement, public FormAssociatedEle
 public:
     virtual ~HTMLFormControlElement();
 
-    HTMLFormElement* form() const { return FormAssociatedElement::form(); }
-
     String formEnctype() const;
     void setFormEnctype(const AtomicString&);
     String formMethod() const;
@@ -145,6 +143,7 @@ private:
     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;