Make the promise settlement type of WindowClient.navigate() nullable (#1253)

The navigate() can be resolved as null if If browsingContext’s Window
object’s environment settings object’s creation URL’s origin is not
the same as the service worker's origin[1].

[1] https://w3c.github.io/ServiceWorker/#ref-for-dom-windowclient-navigate

Author: romandev

docs/index.bs

@@ -981,7 +981,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
         readonly attribute boolean focused;
         [SameObject] readonly attribute FrozenArray<USVString> ancestorOrigins;
         [NewObject] Promise<WindowClient> focus();
-        [NewObject] Promise<WindowClient> navigate(USVString url);
+        [NewObject] Promise<WindowClient?> navigate(USVString url);
       };
     </pre>