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With the DestroyProfileOnBrowserClose experiment, there may be a lot
more use-after-free bugs in ~PrefChangeRegistrar(), so add a comment
explaining this and giving a point-of-contact.

Bug: 1141952
Change-Id: Ie3887151d511fdd965763598d6fc33599cdf60dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2497989
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Commit-Queue: Dominic Battré <battre@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820782}