Return nullable WindowProxy from document.open(url, name, features)

This overload delegates to the window open steps, which can return null.

Fixes #4091.

source

@@ -9236,7 +9236,7 @@ partial interface <dfn id="document" data-lt="">Document</dfn> {
 
   // <span>dynamic markup insertion</span>
   [<span>CEReactions</span>] <span>Document</span> <span data-x="dom-document-open">open</span>(optional DOMString unused1, optional DOMString unused2); // both arguments are <a href="#dom-document-open-unused-arguments">ignored</a>
-  <span>WindowProxy</span> <span data-x="dom-document-open-window">open</span>(USVString url, DOMString name, DOMString features);
+  <span>WindowProxy</span>? <span data-x="dom-document-open-window">open</span>(USVString url, DOMString name, DOMString features);
   [<span>CEReactions</span>] void <span data-x="dom-document-close">close</span>();
   [<span>CEReactions</span>] void <span data-x="dom-document-write">write</span>(DOMString... text);
   [<span>CEReactions</span>] void <span data-x="dom-document-writeln">writeln</span>(DOMString... text);