merge_pr_60230

Ensure the fullscreen element is allowed before setting it sync.

If our document is already fullscreen, we were bypassing the "fullscreen
element allowed" check, causing an svg:text element to become
fullscreen incorrectly.

Differential Revision: https://phabricator.services.mozilla.com/D302736

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2025058
gecko-commit: db8f4f238aa5242445525739fa90efc061f230d0
gecko-commit-git: 9576c85640c495257eb396625db626b596f72f7d
gecko-reviewers: sfarre, dom-core, smaug