[web-animations-2] Add steps for transitioning to/from a scroll timeline. (#5423)

Copies procedures for setting the timeline, current time, start time and starting animations from web-animations-1 and adds steps for transitioning to/from a scroll timeline:
* When transitioning to a scroll timeline, start time is updated to synchronize current time with the scroll position.
* When transitioning from a scroll timeline to a document timeline, current time is preserved.

Author: kevers-google