Flex spec reference that defines how to find the last baseline of flex containers: https://www.w3.org/TR/css-flexbox-1/#flex-baselines
<rdar://problem/100882674>
Pull request: https://github.com/WebKit/WebKit/pull/5248
Committed 255575@main (7957e0705174): <https://commits.webkit.org/255575@main> Reviewed commits have been landed. Closing PR #5248 and removing active labels.