Remove redundant [WillBeGarbageCollected] uses on derived EventTargets.

Source/core/html/track/TextTrack.idl

 /*
  * Copyright (C) 2011 Google Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 10 matching lines @@
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 enum TextTrackMode { "disabled",  "hidden",  "showing" };
 enum TextTrackKind { "subtitles",  "captions",  "descriptions",  "chapters",  "metadata" };
 
 [
     SetWrapperReferenceFrom=owner,
-    WillBeGarbageCollected,
 ] interface TextTrack : EventTarget {
     readonly attribute TextTrackKind kind;
     readonly attribute DOMString label;
     readonly attribute DOMString language;
 
     readonly attribute DOMString id;
 
              attribute TextTrackMode mode;
 
     readonly attribute TextTrackCueList cues;
     readonly attribute TextTrackCueList activeCues;
 
     void addCue(TextTrackCue cue);
     [RaisesException] void removeCue(TextTrackCue cue);
 
              attribute EventHandler oncuechange;
 
     [RuntimeEnabled=WebVTTRegions] readonly attribute VTTRegionList regions;
     [RuntimeEnabled=WebVTTRegions] void addRegion(VTTRegion region);
     [RaisesException, RuntimeEnabled=WebVTTRegions] void removeRegion(VTTRegion region);
 };