Chrome Version: 60.0.3112.34 (post-reboot)
OS: ChromeOS Caroline

What steps will reproduce the problem?
[Not sure what the trigger for this was; maybe an auto-update?]
(1) Sign-in to ChromeOS.
(2) Use it for several days, with sleep/resume cycles.
(3) Sign-out of ChromeOS.

What is the expected result?

Expect that sign-out is pretty much instantaneous, or perhaps takes ~10s if there is an auto-update pending.

What happens instead?

Please use labels and text to provide additional information.

Screen went blank but device stayed powered on, for about 60 seconds, after which the sign-in screen was shown (I looked away so didn't see whether the boot animation was shown).

According to top, the device actually rebooted during that time.

Note that the recent "Critical update" had already been installed on this device.

Not sure what the underlying issue is, to tag appropriately, so CC'ing igo@ in case we have similar regression reports.