[css-scroll-snap] Have ScrollSnapAnimatorState calculate the HashSet of currently snapped boxes immediately
<rdar://problem/104816359>
Pull request: https://github.com/WebKit/WebKit/pull/9400
Committed 259696@main (279d12080527): <https://commits.webkit.org/259696@main> Reviewed commits have been landed. Closing PR #9400 and removing active labels.
Committed 259548.72@safari-7615-branch (355b307a1241): <https://commits.webkit.org/259548.72@safari-7615-branch> Reviewed commits have been landed. Closing PR #372 and removing active labels.