VTTCue: Support 'auto' for line

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

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+enum AutoKeyword { "auto" };
 enum DirectionSetting { "" /* horizontal */, "rl", "lr" };
 enum AlignSetting { "start", "middle", "end", "left", "right" };
 
 [
     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 line;
+    [RaisesException=Setter] attribute (double or AutoKeyword) line;
     [RaisesException=Setter] attribute double position;
     [RaisesException=Setter] attribute double size;
     attribute AlignSetting align;
     attribute DOMString text;
     DocumentFragment getCueAsHTML();
 };