Bug 1867667 - [bidi] Use correct check to identify WindowProxy in the serialization. r=webdriver-reviewers,whimboo

Differential Revision: https://phabricator.services.mozilla.com/D195210

remote/webdriver-bidi/RemoteValue.sys.mjs

@@ -919,7 +919,7 @@ export function serialize(
     }
 
     return serialized;
-  } else if (className === "Window") {
+  } else if (Window.isInstance(value)) {
     const serialized = buildSerialized("window", handleId);
     const window = Cu.unwaiveXrays(value);