When getting the computed style of a grid item that has a trimmed margin, it should be reported as 0
<rdar://problem/106559432>
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/39558
Pull request: https://github.com/WebKit/WebKit/pull/12759
Committed 263008@main (67d0e83ce373): <https://commits.webkit.org/263008@main> Reviewed commits have been landed. Closing PR #12759 and removing active labels.