Implement TextTrackCue::ComputedLinePosition.

This needs some extra infrastructure to be tested in our code. It does work in Gecko due to the extra code I'm landing in https://bugzilla.mozilla.org/show_bug.cgi?id=881976.

I can add the proper infrastructure and other tests when I _finally_ land the overlap avoidance part of the processing model, which I'm almost finished. I think it will be good enough for now just to have the most basic shim in place for the Track stuff.