Bug 1630655 - Move OS.File shutdown forward r=froydnj

As far as I've been able to find digging through source code and
adding logging on try - everything at least intends to do its
writeAtomic calls before xpcom-will-shutdown. So we may as well
make that more explicit.

This won't make OS.File writes air tight. We could still add writes
after this point and they will simply fail. However, that was still
possible before this change anyway.

Depends on D71190

Differential Revision: https://phabricator.services.mozilla.com/D74743

toolkit/components/osfile/modules/osfile_async_front.jsm

@@ -625,7 +625,7 @@ if (SharedAll.Config.DEBUG && Scheduler.launched) {
 const PREF_OSFILE_TEST_SHUTDOWN_OBSERVER =
   "toolkit.osfile.test.shutdown.observer";
 
-AsyncShutdown.webWorkersShutdown.addBlocker(
+AsyncShutdown.xpcomWillShutdown.addBlocker(
   "OS.File: flush pending requests, warn about unclosed files, shut down service.",
   async function() {
     // Give clients a last chance to enqueue requests.