See: https://bugzilla.mozilla.org/show_bug.cgi?id=1649595#c8

This will allow us to replace instances of OS.File.profileBeforeChange.addBlocker with AsyncShutdown.profileBeforeChange.addBlocker

IOUtils now provides a shutdown client for clients to register IO jobs with
that need to run before shutdown finishes. This allows IO jobs to run during
the profileBeforeChange phase.

IOUtils' event queuing has been refactored into a singleton EventQueue class,
which is responsible for running tasks and setting up shutdown blockers. It is
now guarded behind a StaticDataMutex so that is more explicitly clear what is
and is not re-entrant. (The atomic sShutdownFinished cannot be placed behind
the mutex because we need to keep track of that state even when we do not have
an EventQueue.)

Depends on D103972

Backed out for causing mochitest withouth e10s failures at IOUtils

Backout link: https://hg.mozilla.org/integration/autoland/rev/e3a6a6a2318e8f18f11208cdd54c69ad4c116bb8

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=eL9mc_MMSoCQF-Uvrtlfpw.0&resultStatus=success%2Ctestfailed%2Cbusted%2Cexception%2Crunning%2Cpending%2Crunnable&searchStr=linux%2C18.04%2Cx64%2Cdebug%2Cmochitests%2Cwithout%2Ce10s%2Ctest-linux1804-64%2Fdebug-mochitest-chrome-1proc%2Cc1&revision=1f5a77c484b06e785505661ae18022831c8be467

Failure log: https://treeherder.mozilla.org/logviewer?job_id=330364821&repo=autoland&lineNumber=2678

Previously we were intializing IOUtils lazily when we received our first IO
request, but that could run into problems when

1. shutdown starts before submitting any IO requests, which would then fail to
   add shutdown blockers for phases we have already passed; or
2. the first IO request comes from off-main-thread, which can not tell if the
   shutdown blockers are added succesfully.

Now we register an observer to initialize the event queue and add all startup
blockers in the profile-after-change phase. We can then be sure that IOUtils
will have its blockers added before profile-after-change.

Depends on D103973

Backed out for causing xpc failures in mozilla::dom::IOUtils::GetEventQueue

Backout link: https://hg.mozilla.org/integration/autoland/rev/2d2d11b73a7534c1c46dafb4ff367739792ecf77

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=f7OtfCVBT5efXFmZeHacaw.0&resultStatus=testfailed%2Cbusted%2Cexception%2Crunning%2Cpending%2Crunnable&revision=ef2e347ff4f0cf773da52e307c42c644db870252&searchStr=xpc

Failure log android xpc: https://treeherder.mozilla.org/logviewer?job_id=330781771&repo=autoland&lineNumber=1799

Failure log linux xpc: https://treeherder.mozilla.org/logviewer?job_id=330781430&repo=autoland

LE: it seems there are marionette failures too:

https://treeherder.mozilla.org/logviewer?job_id=330781541&repo=autoland&lineNumber=30286

Backed out 2 changesets (Bug 1672431) for causing bustages in IOUtils.cpp
Backout link: https://hg.mozilla.org/integration/autoland/rev/0e114291722e7ef7b01d5ce72ed8638859228f85
Push with failures, failure log.

Backed out 2 changesets (bug 1672431) for Browser-chrome failures with assertion on checkouts/gecko/dom/system/IOUtils.cpp. CLOSED TREE

Log:
https://treeherder.mozilla.org/logviewer?job_id=332752493&repo=autoland&lineNumber=22488

Push with failures:
https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&revision=b9b380d9a847283c6643ebbb82b3838c90a0453c

Backout:
https://hg.mozilla.org/integration/autoland/rev/f025057a87ceeafc84ca4044c527e6cb6d943d06

https://hg.mozilla.org/mozilla-central/rev/33a2d9a28a54
https://hg.mozilla.org/mozilla-central/rev/f0b2f3fad47a