When getting the computed style of a flex item that has a trimmed margin, it should be reported as 0
<rdar://problem/106558991>
Pull request: https://github.com/WebKit/WebKit/pull/11640
Committed 262081@main (81223299d3fb): <https://commits.webkit.org/262081@main> Reviewed commits have been landed. Closing PR #11640 and removing active labels.