Bugzilla

The rationale here is related to https://bugzilla.mozilla.org/show_bug.cgi?id=1789126#c22 ; tl;dr is:
 - we hit a reject
 - everything suggest this is because of `GeckoChildProcessHost::Destroy` that rejects when we are in the begining of starting a process
 - we have `IsShutdown()` guarding on our utility process manager
 - this depends on a boolean updated when `xpcom-shutdown` observer is delivered to us
 - the test reproducing are tests where I already spotted raciness around starting a process when shutting down

So my take is we enter a small window of possible race between the `GeckoChildProcessHost::Destroy` and the moment we receive the observer, hence we get rejected by `Destroy` but we still dont know we are in shutdown. Moving to `AppShutdown` would, I hope, avoid that.