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When the user session is stopped, we don't have access to user logs
anymore, yet it's important to have information on the reason why the
session was stopped.

BUG=chromium:1014538
TEST=Check that reason is logged at sign out.

Change-Id: I2869b000db5750cafba8566f8d1e13cb1c2369c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1871673
Commit-Queue: Igor <igorcov@chromium.org>
Reviewed-by: Igor <igorcov@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719068}