* Add a default-true pref to provide an escape hatch allowing us to revert to previous behavior * in which recently-closed tabs are per-window, * and undoing closed tabs restores them to the window they were closed from. * Ensure we set the pref for tests which depend on its value * Add some spot-checks in tests with the pref off Differential Revision: https://phabricator.services.mozilla.com/D179574