Deal with edge case in the code from bug 1891335.
Apparently we might not create a subdoc frame for unless it's
inside .
Let's treat it as fully hidden.
Differential Revision: https://phabricator.services.mozilla.com/D221943
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1917361
gecko-commit: 36af09c3c6fa2111176acc433edc074dda75209d
gecko-reviewers: smaug