This causes postMessage and other window functions to be broken in layout tests with site isolation enabled.
<rdar://problem/115753929>
Pull request: https://github.com/WebKit/WebKit/pull/17947
Committed 268182@main (040fd20a08c9): <https://commits.webkit.org/268182@main> Reviewed commits have been landed. Closing PR #17947 and removing active labels.