[touch-events] Index TouchList as array

myTouchList[123] is allowed as an alias for myTouchList.item(123).

https://developer.mozilla.org/en-US/docs/Web/API/TouchList
and
http://www.w3.org/TR/touch-events/#idl-def-TouchList

Tested the proposed change on ff mobile, works fine. Didn't check the -test.ts file, should be fine too.