When getting the computed style of a flex item that has a trimmed margin, it should be reported as 0
<rdar://problem/106559532>
Pull request: https://github.com/WebKit/WebKit/pull/11741
Committed 262663@main (176479519f31): <https://commits.webkit.org/262663@main> Reviewed commits have been landed. Closing PR #11741 and removing active labels.
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/39415