This is in anticipation of adding Animation::SetTimeline(). Once we set the
timeline out of band, there's a chance that getting it could return null so
this patch makes the WebIDL and method name reflect that.
This is not strictly necessary yet but we will want to implement methods
like GetAnimations() on the base class, AnimationTimeline, so we may as well do
that now rather than adding that code to DocumentTimeline and moving it later.
