Make VTTRegion.scroll an enum ScrollSetting
Remove dead code throwing SyntaxError in enum setters

https://www.w3.org/Bugs/Public/show_bug.cgi?id=22967
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22968
Merge pull request #28 from foolip/enum-syntax-error

Make VTTRegion.scroll an enum ScrollSetting

Remove dead code throwing SyntaxError in enum setters
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22967
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22968