When getting the computed style of a grid item that has a trimmed margin, it should be reported as 0
<rdar://problem/106559597>
Pull request: https://github.com/WebKit/WebKit/pull/12749
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/39555
Committed 263006@main (53332e5f7256): <https://commits.webkit.org/263006@main> Reviewed commits have been landed. Closing PR #12749 and removing active labels.