In order to advance fast shutdown to stage 3, we need to clear out all late write telemetry coming in. Some of that appears to be from xulstore::persist. This bug tracks ensuring that we flush all writes before we turn on late writes or do a fast shutdown.

Some background information on why we want to flush writes: we're
currently working on a "fast shutdown" project, which will simply
call _exit(0) once a certain phase (controlled via pref) in
shutdown is hit. This will allow us to skip a lot of ceremony,
like freeing memory, removing listeners on observer notifications,
etc. However, we don't want to be cavalier about data loss, so
for the time being we want to ensure that all file writes which we
previously had some assurance would complete will continue to
complete. This means explicitly calling them immediately before a
fast shutdown. We also want to call these before we turn on late
write checking, which we control via another pref. This late write
checking surfaces writes via telemetry which we didn't notice with
local testing. However we want to ensure that we flush our writes
before this point so we don't have lots of noise in the telemetry.

Lina I'm requesting review from you since it looks like you did the
reviews on most of the initial code. However, please let me know if
there's someone more suitable.

Depends on D75389

https://hg.mozilla.org/mozilla-central/rev/952c7a8bc4bc