It looks like Safari used to expose this via AXARIAControls, which is
why we originally supported this that way. However, they now adhere to
AAM along with Chrome, which says to use AXLinkedUIElements. VoiceOver won't
announce accs exposed via AXARIAControls any more :( I've
verified this manually in addition to running the automated tests that
this patch updates
Differential Revision: https://phabricator.services.mozilla.com/D311962