LibWeb: Add MessageEvent.source

Author: https://github.com/Lubrsi
Commit: 1607b2c978
Pull-request: https://github.com/SerenityOS/serenity/pull/21823
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/shannonbooth
Reviewed-by: https://github.com/trflynn89

Userland/Libraries/LibWeb/HTML/MessageEvent.cpp

@@ -24,6 +24,7 @@ MessageEvent::MessageEvent(JS::Realm& realm, FlyString const& event_name, Messag
     , m_data(event_init.data)
     , m_origin(event_init.origin)
     , m_last_event_id(event_init.last_event_id)
+    , m_source(event_init.source)
 {
 }
 
@@ -41,4 +42,12 @@ void MessageEvent::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_data);
 }
 
+Variant<JS::Handle<WindowProxy>, JS::Handle<MessagePort>, Empty> MessageEvent::source() const
+{
+    if (!m_source.has_value())
+        return Empty {};
+
+    return m_source.value().downcast<JS::Handle<WindowProxy>, JS::Handle<MessagePort>>();
+}
+
 }