Inherit EventTarget interface instead of duplicating its code

Source/modules/mediastream/RTCDTMFSender.idl

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 [
     NoInterfaceObject,
-    ActiveDOMObject,
-    EventTarget
-] interface RTCDTMFSender {
+    ActiveDOMObject
+] interface RTCDTMFSender : EventTarget {
     readonly attribute boolean canInsertDTMF;
     readonly attribute MediaStreamTrack track;
     readonly attribute DOMString toneBuffer;
     readonly attribute long duration;
     readonly attribute long interToneGap;
 
     [RaisesException] void insertDTMF(DOMString tones, optional long duration, optional long interToneGap);
 
     attribute EventListener ontonechange;
-
-    // EventTarget interface
-    void addEventListener(DOMString type,
-                          EventListener listener,
-                          optional boolean useCapture);
-    void removeEventListener(DOMString type,
-                             EventListener listener,
-                             optional boolean useCapture);
-    [RaisesException] boolean dispatchEvent(Event event);
 };