Change textContent fix for IE8 to use innerText.
Change textContent fix for IE8 to use innerText.

'innerHTML' won't work in this case as some of the nodes
we're working with can be text nodes which don't support
'innerHTML'. Instead we can use 'innerText' if
'textContent' does not exist on the node as IE8/7 support
this method.

gkatsev/vtt.js,gkatsev/vtt.js,mozilla/vtt.js,mozilla/vtt.js