When getting the computed style of a flex item that has a trimmed margin, it should be reported as 0
<rdar://problem/106559023>
Pull request: https://github.com/WebKit/WebKit/pull/11657
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/39417
Committed 262700@main (eeb7fe30a25b): <https://commits.webkit.org/262700@main> Reviewed commits have been landed. Closing PR #11657 and removing active labels.