Have ElementTraversal / NodeTraversal's next() methods take a reference

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 e5d60479e5cfc15bc380ea845ed92ab6050b1573..a241b8be561691ee48f823a0b8b822a27ad5e2e3 100644
--- a/Source/core/html/HTMLFormControlElement.h
+++ b/Source/core/html/HTMLFormControlElement.h
@@ -203,6 +203,12 @@ inline const HTMLFormControlElement* toHTMLFormControlElement(const FormAssociat
     return static_cast<const HTMLFormControlElement*>(control);
 }
 
+inline const HTMLFormControlElement& toHTMLFormControlElement(const FormAssociatedElement& control)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(control.isFormControlElement());
+    return static_cast<const HTMLFormControlElement&>(control);
+}
+
 } // namespace
 
 #endif