LibWeb: Make MessageEvents from {Window,MessagePort}.postMessage trusted

The MessagePort one in particular is required by Cloudflare Turnstile,
as the method it takes to run JS in a worker is to `eval` the contents
of `MessageEvent.data`. However, it will only do this if
`MessageEvent.isTrusted` is true, `MessageEvent.origin` is the empty
string and `MessageEvent.source` is `null`.

The Window version is a quick fix whilst in the vicinity, as its
MessageEvent should also be trusted.

Libraries/LibWeb/HTML/Window.cpp

@@ -1165,6 +1165,7 @@ WebIDL::ExceptionOr<void> Window::window_post_message_steps(JS::Value message, W
         message_event_init.ports = move(new_ports);
 
         auto message_event = MessageEvent::create(target_realm, EventNames::message, message_event_init);
+        message_event->set_is_trusted(true);
         dispatch_event(message_event);
     }));