Sync audio/video/text track-related interfaces with the specs

Source/core/html/track/VideoTrackList.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// https://html.spec.whatwg.org/#videotracklist
+
 [
     RuntimeEnabled=AudioVideoTracks,
     SetWrapperReferenceFrom=owner,
 ] interface VideoTrackList : EventTarget {
   readonly attribute unsigned long length;
   getter VideoTrack (unsigned long index);
   VideoTrack? getTrackById(DOMString id);
   readonly attribute long selectedIndex;
 
   attribute EventHandler onchange;
   attribute EventHandler onaddtrack;
   attribute EventHandler onremovetrack;
 };