Investigate UNC path restrictions failure in Tor Browser 8.0a9

Filippo Cavallarin reported that the UNC path restrictions that got made available through https://bugzilla.mozilla.org/show_bug.cgi?id=1413868 are not working in Tor Browser 8.0a9. Surprisingly, Tor Browser 7.5.6 and Firefox 60.1.0 ESR are fine.