Load WebArchive with non-persistent datastore
<rdar://problem/122290562>
Pull request: https://github.com/WebKit/WebKit/pull/23843
Pull request: https://github.com/WebKit/WebKit/pull/24208
Committed 274565@main (a350c1e9191a): <https://commits.webkit.org/274565@main> Reviewed commits have been landed. Closing PR #23843 and removing active labels.