So it'd be easier to introduce view timelines by overriding ComputeOffsets().
Basically, we use offsets to control the animation progress.
For scroll-timeline, it's simply 0 to the scroll range.
For view-timeline, its progress is between 0% and 100% if the subject element
is in the view. Otherwise, its progress is outside the range. We will
implement the offset calculation of view-timeline in Bug 1737920.