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Because the fork server is used to remotely `waitpid` child processes, and this can be done until very late in shutdown, this patch extends the fork server's lifetime. This is a little complicated: it requires some changes to `GeckoChildProcessHost::Destroy` to allow it to work after the I/O thread is no longer accepting runnables. Differential Revision: https://phabricator.services.mozilla.com/D240715