LibWeb: Add simple ad-hoc version of window.postMessage()

This allows us to use the wpt.live copy of the ACID3 test, which is kept
updated, unlike the acidtests.org version.

Userland/Libraries/LibWeb/DOM/Window.h

@@ -102,6 +102,8 @@ public:
 
     Window* parent();
 
+    DOM::ExceptionOr<void> post_message(JS::Value, String const& target_origin);
+
 private:
     explicit Window(Document&);