Inherit EventTarget interface instead of duplicating its code

Source/core/dom/MessagePort.idl

 /*
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  * Copyright (C) 2011 Google Inc. All Rights Reserved.
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 [
-    ActiveDOMObject,
-    EventTarget
-] interface MessagePort {
+    ActiveDOMObject
+] interface MessagePort : EventTarget {
 // We need to have something as an ObjC binding, because MessagePort is used in MessageEvent, which already has one,
 // but we don't want to actually expose the API while it is in flux.
     [Custom, RaisesException] void postMessage(any message, optional Array messagePorts);
 
     void start();
     void close();
 
     // event handler attributes
     attribute EventListener onmessage;
-
-    // EventTarget interface
-    void addEventListener(DOMString type, 
-                          EventListener listener, 
-                          optional boolean useCapture);
-    void removeEventListener(DOMString type, 
-                             EventListener listener, 
-                             optional boolean useCapture);
-    [RaisesException] boolean dispatchEvent(Event evt);
 };