Do it in a bit more straight-forward way than pre-regression, by just checking the chrome flag before saving the persistent attribute. This really undoes the purpose of the regressor, which was fixing issues with bug 2054183 (since we go back to restoring the sizemode attribute from the xul store...). But turns out that since then we've removed the window gap stuff that caused that issue (plus even with the gaps we decided to remove the gaps from popups altogether), so we might not really care about that anymore. Differential Revision: https://phabricator.services.mozilla.com/D323778