This is closer to what the spec instructs us to do, and matches how
associations are maintained in the timelines. Also note that the removed
destructor logic is not necessary since we visit the associated
animations anyways.
If a DOM::Element has an animation-name property, then in addition to
remembering where it came from, it will also remember the
Animations::Animation object that was created for it. This allows
StyleComputer to cancel that animation if the animation-name property
changes as well as to apply any changes required (for example, if
animation-play-state changes from "running" to "paused", it needs to
call .pause() on the animation).
With this commit, we are finally running animations off of the web
animations spec! A lot of the work StyleComputer is doing is now done
elsewhere. For example, fill-forward animations are handled by
Animation::is_relevant() returning true in the after phase, meaning the
"active_state_if_fill_forward" map is no longer needed.
