LibWeb: Add the lastEventId IDL attribute to MessageEvent

Userland/Libraries/LibWeb/HTML/MessageEvent.h

@@ -23,6 +23,7 @@ public:
 
     JS::Value data() const { return m_data; }
     String const& origin() const { return m_origin; }
+    String const& last_event_id() const { return m_last_event_id; }
 
 protected:
     MessageEvent(const FlyString& event_name, JS::Value data, String origin)
@@ -34,6 +35,7 @@ protected:
 
     JS::Value m_data;
     String m_origin;
+    String m_last_event_id;
 };
 
 }

Userland/Libraries/LibWeb/HTML/MessageEvent.idl

@@ -2,5 +2,7 @@ interface MessageEvent : Event {
 
     readonly attribute any data;
     readonly attribute USVString origin;
-
+    readonly attribute DOMString lastEventId;
+    // FIXME: readonly attribute MessageEventSource? source;
+    // FIXME: readonly attribute FrozenArray<MessagePort> ports;
 };