WebVTT: Support 'auto' text position

Source/core/html/track/vtt/VTTCue.idl

 /*
  * Copyright (c) 2013, Opera Software ASA. 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 23 matching lines @@
 [
     Constructor(double startTime, double endTime, DOMString text),
     ConstructorCallWith=Document,
     SetWrapperReferenceFrom=owner,
     TypeChecking=Unrestricted,
 ] interface VTTCue : TextTrackCue {
     [RuntimeEnabled=WebVTTRegions] attribute DOMString regionId;
     attribute DirectionSetting vertical;
     attribute boolean snapToLines;
     [RaisesException=Setter] attribute (double or AutoKeyword) line;
-    [RaisesException=Setter] attribute double position;
+    [RaisesException=Setter] attribute (double or AutoKeyword) position;
     [RaisesException=Setter] attribute double size;
     attribute AlignSetting align;
     attribute DOMString text;
     DocumentFragment getCueAsHTML();
 };