Inherit EventTarget interface instead of duplicating its code

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 85514c22d0b3e30ba05b05132e6871536a157a4f..18e728491a62308e8717e401c82e81d135e9dfec 100644
--- a/Source/core/dom/MessagePort.idl
+++ b/Source/core/dom/MessagePort.idl
@@ -26,9 +26,8 @@
  */
 
 [
-    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);
@@ -38,14 +37,5 @@
 
     // 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);
 };