I think it's confusing to have both currentTime and getCurrentTime.

Would it be possible to:

1. Move this to the ScrollTimeline interface, (Update: I see now that this is proposed to be used for DocumentTimeline too--I still don't quite follow the use cases but assuming we want to do that, I guess it should stay on AnimationTimeline)
2. Rename it to getCurrentTimeForRange, and
3. Make the rangeName argument non-optional

?