Use undefined union for window.event

Author: saschanaz

dom.bs

@@ -776,7 +776,7 @@ incapable of setting {{Event/composed}}. It has to be supported for legacy conte
 
 <pre class=idl>
 partial interface Window {
-  [Replaceable] readonly attribute any event; // legacy
+  [Replaceable] readonly attribute (Event or undefined) event; // legacy
 };
 </pre>