VTTCue: Support 'auto' for line

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

Index: Source/core/html/track/vtt/VTTCue.idl
diff --git a/Source/core/html/track/vtt/VTTCue.idl b/Source/core/html/track/vtt/VTTCue.idl
index 08ed7997c81c3506484357b86cb1cb1199d9b922..7401883d15ab1a17f5c90f6f8e6f9cad9ca5d786 100644
--- a/Source/core/html/track/vtt/VTTCue.idl
+++ b/Source/core/html/track/vtt/VTTCue.idl
@@ -27,6 +27,7 @@
  * OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+enum AutoKeyword { "auto" };
 enum DirectionSetting { "" /* horizontal */, "rl", "lr" };
 enum AlignSetting { "start", "middle", "end", "left", "right" };
 
@@ -39,7 +40,7 @@ enum AlignSetting { "start", "middle", "end", "left", "right" };
     [RuntimeEnabled=WebVTTRegions] attribute DOMString regionId;
     attribute DirectionSetting vertical;
     attribute boolean snapToLines;
-    [RaisesException=Setter] attribute double line;
+    attribute (double or AutoKeyword) line;
     [RaisesException=Setter] attribute double position;
     [RaisesException=Setter] attribute double size;
     attribute AlignSetting align;