Under unknown circumstances, we can end up running chrome
JS during thread manager shutdown. Sometimes this ends up
trying to load new JSMs, but gJarHandler has already been
cleared, leading to a crash.

To avoid this and other issues, this patch forbids the
importing of new JSMs after we're late enough in shutdown
to have cleared the ClearOnShutdown pointers. I allow the
importing of JSMs that have already been loaded, as that
seems like it should be okay.

Differential Revision: https://phabricator.services.mozilla.com/D89477