LibWeb/HTML: Port Window.parent to IDL

Author: https://github.com/linusg
Commit: c6353ac8cd
Pull-request: https://github.com/SerenityOS/serenity/pull/17752
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/HTML/Window.idl

@@ -18,6 +18,7 @@ interface Window : EventTarget {
     [Replaceable] readonly attribute WindowProxy frames;
     [Replaceable] readonly attribute unsigned long length;
     [LegacyUnforgeable] readonly attribute WindowProxy? top;
+    [Replaceable] readonly attribute WindowProxy? parent;
 
     // the user agent
     readonly attribute Navigator navigator;