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

Source/core/html/HTMLLegendElement.h

Index: Source/core/html/HTMLLegendElement.h
diff --git a/Source/core/html/HTMLLegendElement.h b/Source/core/html/HTMLLegendElement.h
index 4a8c2fa6caa3943aaf9d3629f1bbb42e6970de63..74290a8ea33c715dc70752d899b7c0dbc40b3f33 100644
--- a/Source/core/html/HTMLLegendElement.h
+++ b/Source/core/html/HTMLLegendElement.h
@@ -34,6 +34,8 @@ class HTMLLegendElement FINAL : public HTMLElement {
 public:
     static PassRefPtr<HTMLLegendElement> create(Document&);
 
+    HTMLFormElement* form() const;
+
 private:
     explicit HTMLLegendElement(Document&);
 
@@ -42,7 +44,6 @@ private:
 
     virtual void accessKeyAction(bool sendMouseEvents);
     virtual void focus(bool restorePreviousSelection, FocusDirection) OVERRIDE;
-    virtual HTMLFormElement* virtualForm() const OVERRIDE;
 };
 
 DEFINE_NODE_TYPE_CASTS(HTMLLegendElement, hasTagName(HTMLNames::legendTag));