https://feedback.corp.google.com/#/Report/54725165326 

CHROME VERSION: 57.0.2987.85 beta
CHROMEOS_AUSERVER: <URL: 3>
CHROMEOS_RELEASE_BOARD: kevin-signed-mpkeys
CHROMEOS_RELEASE_DESCRIPTION: 9202.43.0 (Official Build) beta-channel kevin
CHROMEOS_RELEASE_TRACK: beta-channel
CHROMEOS_RELEASE_VERSION: 9202.43.0

Confirm the exact timing of the reboot from POWERD.PREVIOUS:

[0309/124417:INFO:daemon.cc(1148)] Got RequestRestart message from :1.26
[0309/124417:INFO:display_power_setter.cc(80)] Asking Chrome to turn all displays off
[0309/124417:INFO:internal_backlight_controller.cc(674)] Setting brightness to 0 (0%) over 0 ms
[0309/124417:INFO:daemon.cc(1504)] Restarting, reason: system-update
[0309/124417:INFO:main.cc(240)] Launching "/usr/bin/powerd_setuid_helper --action=reboot"
[0309/124417:INFO:suspend_delay_controller.cc(76)] Unregistering suspend delay 90308609 (shill) on behalf of :1.10
[0309/124417:INFO:suspend_delay_controller.cc(76)] Unregistering dark suspend delay 90341377 (shill) on behalf of :1.10
[0309/124417:INFO:daemon.cc(1431)] Session state changed to stopped
[0309/124417:INFO:state_controller.cc(757)] Updated settings: dim=5m screen_off=6m lock=6m10s idle_warn=0s idle=10m (suspend) lid_closed=no-op use_audio=1 use_video=1
[0309/124420:INFO:suspend_delay_controller.cc(76)] Unregistering suspend delay 90308613 (session_manager) on behalf of :1.6
[0309/124421:INFO:suspend_delay_controller.cc(123)] Unregistering suspend delay 90308612 (chrome) due to D-Bus client :1.348 going away
[0309/124421:INFO:suspend_delay_controller.cc(123)] Unregistering dark suspend delay 90341379 (chrome) due to D-Bus client :1.348 going away

--

Check SYSLOG.  See these things:

2017-03-09T12:44:17.655224-08:00 INFO session_manager[1449]: [INFO:browser_job.cc(157)] Terminating process: 
2017-03-09T12:44:17.656030-08:00 INFO session_manager[1449]: [INFO:system_utils_impl.cc(110)] Sending 15 to 3005 as 1000
...
2017-03-09T12:44:20.825232-08:00 WARNING session_manager[1449]: [WARNING:browser_job.cc(165)] Aborting child process 3005's process group 3 seconds after sending signal
2017-03-09T12:44:20.825319-08:00 INFO session_manager[1449]: [INFO:browser_job.cc(149)] Terminating process group: Browser took more than 3 seconds to exit after signal.
2017-03-09T12:44:20.825369-08:00 INFO session_manager[1449]: [INFO:system_utils_impl.cc(110)] Sending 6 to -3005 as 1000
...
2017-03-09T12:44:20.875199-08:00 WARNING crash_reporter[5911]: [user] Received crash notification for chrome[3024] sig 6, user 1000 (ignoring call by kernel - chrome crash; waiting for chrome to call us directly)
...
2017-03-09T12:44:21.170887-08:00 WARNING crash_reporter[5925]: Received crash notification for chrome[3005] user 1000 (called directly)
...
2017-03-09T12:44:21.179391-08:00 WARNING crash_reporter[5927]: Received crash notification for chrome-crash-unknown-process[3290] user 1000 (called directly)
...
2017-03-09T12:44:21.204327-08:00 WARNING crash_reporter[5928]: [user] Received crash notification for chrome[3050] sig 6, user 1000 (ignoring call by kernel - chrome crash; waiting for chrome to call us directly)
...
2017-03-09T12:44:21.266571-08:00 WARNING crash_reporter[5936]: Received crash notification for chrome[3050] user 1000 (called directly)
...
2017-03-09T12:44:21.284474-08:00 WARNING crash_reporter[5946]: [user] Received crash notification for chrome[3084] sig 6, user 1000 (ignoring call by kernel - chrome crash; waiting for chrome to call us directly)
2017-03-09T12:44:21.302586-08:00 WARNING crash_reporter[5945]: Received crash notification for chrome-crash-unknown-process[3409] user 1000 (called directly)
...
2017-03-09T12:44:21.312069-08:00 WARNING crash_reporter[5954]: [user] Received crash notification for chrome[3005] sig 6, user 1000 (ignoring call by kernel - chrome crash; waiting for chrome to call us directly)
...
2017-03-09T12:44:21.491669-08:00 CRIT ui-post-stop-unkillable[5972]:  3005 Z<sl [chrome] <defunct>
2017-03-09T12:44:21.491680-08:00 CRIT ui-post-stop-unkillable[5972]:  3050 Z<l  [chrome] <defunct>
2017-03-09T12:44:21.491685-08:00 CRIT ui-post-stop-unkillable[5972]:  5931 Zs   [gpu-process_cra] <defunct>

--

Cheat and search crash for crashes with the same client ID and PID 3005:

ClientID='<ID>' OMIT RECORD IF SOME(ProductData.key='pid' AND ProductData.value='3005') = 0

Find report f321193920000000

--

In report f321193920000000, we can find Chrome log for the appropriate timeframe (chrome_20170309-124331):

[3005:3005:0309/124417.490703:ERROR:device_event_log_impl.cc(140)] [12:44:17.490] Network: network_state_handler.cc:920 Default network in unexpected state: v1904-5G (/service/0)State: idle
[3005:3005:0309/124417.503725:VERBOSE1:arc_net_host_impl.cc(629)] No default network
[3005:3005:0309/124417.519750:INFO:remote_commands_invalidator.cc(45)] Shutdown RemoteCommandsInvalidator.
[3005:3005:0309/124417.519889:INFO:remote_commands_invalidator.cc(68)] Stopping RemoteCommandsInvalidator.
[3005:3005:0309/124417.520152:INFO:remote_commands_invalidator.cc(167)] Unregister RemoteCommandsInvalidator.
[3005:3005:0309/124417.670605:ERROR:shill_service_client.cc(48)] Failed to call org.chromium.shill.Service.GetProperties for: /service/648: org.freedesktop.DBus.Error.ServiceUnknown: The name org.chromium.flimflam was not provided by any .service files
[3005:3005:0309/124417.670985:ERROR:shill_service_client.cc(48)] Failed to call org.chromium.shill.Service.GetProperties for: /service/649: org.freedesktop.DBus.Error.ServiceUnknown: The name org.chromium.flimflam was not provided by any .service files
[3005:3005:0309/124417.671206:ERROR:shill_service_client.cc(48)] Failed to call org.chromium.shill.Service.GetProperties for: /service/650: org.freedesktop.DBus.Error.ServiceUnknown: The name org.chromium.flimflam was not provided by any .service files
[3005:3005:0309/124417.671342:ERROR:shill_service_client.cc(48)] Failed to call org.chromium.shill.Service.GetProperties for: /service/651: org.freedesktop.DBus.Error.ServiceUnknown: The name org.chromium.flimflam was not provided by any .service files
[3005:3005:0309/124417.671478:ERROR:shill_service_client.cc(48)] Failed to call org.chromium.shill.Service.GetProperties for: /service/647: org.freedesktop.DBus.Error.ServiceUnknown: The name org.chromium.flimflam was not provided by any .service files
[3005:3005:0309/124417.671615:ERROR:shill_service_client.cc(48)] Failed to call org.chromium.shill.Service.GetProperties for: /service/652: org.freedesktop.DBus.Error.ServiceUnknown: The name org.chromium.flimflam was not provided by any .service files
[3005:3005:0309/124417.671809:ERROR:shill_service_client.cc(48)] Failed to call org.chromium.shill.Service.GetProperties for: /service/653: org.freedesktop.DBus.Error.ServiceUnknown: The name org.chromium.flimflam was not provided by any .service files
[3005:3005:0309/124418.013036:ERROR:shill_service_client.cc(48)] Failed to call org.chromium.shill.Service.GetProperties for: /service/654: org.freedesktop.DBus.Error.ServiceUnknown: The name org.chromium.flimflam was not provided by any .service files
[3005:3005:0309/124418.056876:VERBOSE1:arc_bridge_host_impl.cc(211)] Mojo connection lost
[3005:3005:0309/124418.060980:VERBOSE1:arc_policy_bridge.cc(345)] ArcPolicyBridge::OnPolicyInstanceClosed
[3005:3042:0309/124419.515791:ERROR:connection_factory_impl.cc(386)] Failed to connect to MCS endpoint with error -131
[3005:3005:0309/124419.521672:WARNING:feedback_uploader_delegate.cc(58)] FEEDBACK: Submission to feedback server (https://www.google.com/tools/feedback/chrome/__submit) status: No connection to server.
[3005:3048:0309/124421.086478:ERROR:crash_handler_host_linux.cc(435)] Failed to write crash dump for pid 3290
[3005:3048:0309/124421.087143:ERROR:crash_handler_host_linux.cc(435)] Failed to write crash dump for pid 3409
[3005:3048:0309/124421.097783:ERROR:crash_handler_host_linux.cc(435)] Failed to write crash dump for pid 3347


So it sorta looks like Chrome was busy at the time of the restart and didn't respond to the request to exit...