`topDocument()` will use the document of the root frame when site isolation is enabled, so we fail an allowsFirstPartyForCookies check in the network process. Use `Document::topOrigin()` instead.
<rdar://problem/126300675>
Pull request: https://github.com/WebKit/WebKit/pull/27167
Committed 277418@main (b573c0f764bf): <https://commits.webkit.org/277418@main> Reviewed commits have been landed. Closing PR #27167 and removing active labels.