Inherit EventTarget interface instead of duplicating its code

Source/core/html/track/TextTrack.idl

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 [
-    EnabledAtRuntime=videoTrack,
-    EventTarget
-] interface TextTrack {
+    EnabledAtRuntime=videoTrack
+] interface TextTrack : EventTarget {
     readonly attribute DOMString kind;
     readonly attribute DOMString label;
     readonly attribute DOMString language;
 
     attribute DOMString mode;
 
     readonly attribute TextTrackCueList cues;
     readonly attribute TextTrackCueList activeCues;
              attribute EventListener oncuechange;
 
     void addCue(TextTrackCue cue);
     [RaisesException] void removeCue(TextTrackCue cue);
 
     [Conditional=WEBVTT_REGIONS] readonly attribute TextTrackRegionList regions;
     [Conditional=WEBVTT_REGIONS] void addRegion(TextTrackRegion region);
     [RaisesException, Conditional=WEBVTT_REGIONS] void removeRegion(TextTrackRegion region);
-
-    // 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);
 };