We are doing the following things here:
1. Rename ScrollTimelineSet to ProgressTimelineScheduler because this
class is used to schedule animations with progress timelines, including
scroll timelines and view timelines.
2. Drop the element property usage and let ElementAnimationData store
ProgressTimelineScheduler.
3. We avoid using the generated content in ScrollTimeline::Scroller.
Instead, we use a pair of Element and PseudoStyleType to represent
ScrollTimeline::Scroller.
We hit the assertion because the generatd content may change and so we
shouldn't use it as the ScrollTimeline::Scroller.
Differential Revision: https://phabricator.services.mozilla.com/D172610
