Sync the MessageChannel and MessagePort interfaces with the spec

Source/core/dom/MessagePort.idl

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * Copyright (C) 2011 Google Inc. All Rights Reserved.
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+// https://html.spec.whatwg.org/#message-ports
+
 [
     ActiveDOMObject,
+    Exposed=(Window,Worker),
 ] interface MessagePort : EventTarget {
     [Custom, RaisesException] void postMessage(any message, optional sequence<Transferable> transfer);
-
     void start();
     void close();
 
-    // event handler attributes
+    // event handlers
     attribute EventHandler onmessage;
 };