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This allows us to CommitPendingWrite() in PostMainMessageLoopRun(),
while it is still safe to do so, and avoid crashes on shutdown.

Previously, PrefService was doing non-trivial work during
HeadlessContentMainDelegate::~HeadlessContentMainDelegate() that
ended up with SIGABRT on MacOS.

Drive-by: also SetEncryptionKey() in network service on Mac,
similar to Windows.

Bug: 1050905
TBR: gab@chromium.org
Change-Id: I70e632d5f3a03a68b9ef90458315e2c2a516e1f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2634681
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844378}