When getting the computed style of a grid item that has a trimmed margin, it should be reported as 0
<rdar://problem/106559562>
Pull request: https://github.com/WebKit/WebKit/pull/12751
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/39553
Committed 263002@main (7e8288073ff2): <https://commits.webkit.org/263002@main> Reviewed commits have been landed. Closing PR #12751 and removing active labels.