[css-scroll-snap] Add check for element when setting scroll snap id and isFocused for render box
rdar://100652939
Pull request: https://github.com/WebKit/WebKit/pull/4960
Committed 255106@main (2e321fec72f9): <https://commits.webkit.org/255106@main> Reviewed commits have been landed. Closing PR #4960 and removing active labels.