Enable upgrading mixed content
<rdar://problem/122385309>
Pull request: https://github.com/WebKit/WebKit/pull/23919
Committed 274826@main (a1ffadc325c1): <https://commits.webkit.org/274826@main> Reviewed commits have been landed. Closing PR #23919 and removing active labels.
*** Bug 258632 has been marked as a duplicate of this bug. ***