For threaded animation resolution, we introduced an `AcceleratedTimeline` class that did the work of updating accelerated effect stacks for targeted elements that were eligible to run their animation effects in the UI process. To support scroll-driven animations using threaded animation resolution, we will need to send animation timelines over to the UI process and will want to use the term `AcceleratedTimeline` for this purpose. So we should rename `AcceleratedTimeline` to something more appropriate. Additionally, while refactoring, we should move it off `Document` and onto `AnimationTimelinesController` which is already held by `Document`.