Sync the MessageChannel and MessagePort interfaces with the spec

Source/core/dom/MessagePort.idl

Index: Source/core/dom/MessagePort.idl
diff --git a/Source/core/dom/MessagePort.idl b/Source/core/dom/MessagePort.idl
index 70e5f0917a1abb36cee709e9cceee73a30295147..234e880c45f6768721fe8946860c1de4b9657032 100644
--- a/Source/core/dom/MessagePort.idl
+++ b/Source/core/dom/MessagePort.idl
@@ -25,14 +25,16 @@
  *
  */
 
+// 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;
 };