Yep, slave finished the build, got the signal to reboot but it never did.

From the twistd.log:

2017-06-21 02:14:10-0700 [-] command finished with signal None, exit code 0, elapsedTime: 3453.991000
2017-06-21 02:14:10-0700 [-] SlaveBuilder.commandComplete <buildslave.commands.shell.SlaveShellCommand instance at 0x055D6698>
2017-06-21 02:14:10-0700 [Broker,client] Setting needs_reboot flag
2017-06-21 02:14:10-0700 [Broker,client] slave shutting down on command from master
2017-06-21 02:14:10-0700 [-] firing system event shutdown
2017-06-21 02:14:10-0700 [-] calling before trigger: <bound method MultiService.stopService of <twisted.application.service.MultiService instance at 0x052B9468>>
2017-06-21 02:14:10-0700 [-] stopping service <buildslave.bot.BuildSlave instance at 0x052B9558>
2017-06-21 02:14:10-0700 [-] stopping service <twisted.application.internet.TCPClient instance at 0x052B96E8>
2017-06-21 02:14:10-0700 [-] stopped service <twisted.application.internet.TCPClient instance at 0x052B96E8>
2017-06-21 02:14:10-0700 [-] stopping service <buildslave.bot.Bot instance at 0x052B9580>
2017-06-21 02:14:10-0700 [-] stopping service <SlaveBuilder 'Win x64 Builder' at 88865808>
2017-06-21 02:14:10-0700 [-] stopped service <SlaveBuilder 'Win x64 Builder' at 88865808>
2017-06-21 02:14:10-0700 [-] stopped service <buildslave.bot.Bot instance at 0x052B9580>
2017-06-21 02:14:10-0700 [-] stopped service <buildslave.bot.BuildSlave instance at 0x052B9558>
2017-06-21 02:14:10-0700 [-] calling before trigger: <bound method TimerService.stopService of <twisted.application.internet.TimerService instance at 0x05312418>>
2017-06-21 02:14:10-0700 [-] calling during/after trigger: <bound method SelectReactor.crash of <twisted.internet.selectreactor.SelectReactor object at 0x04F3D390>>
2017-06-21 02:14:10-0700 [-] calling during/after trigger: <bound method SelectReactor.disconnectAll of <twisted.internet.selectreactor.SelectReactor object at 0x04F3D390>>
2017-06-21 02:14:10-0700 [Broker,client] lost remote
2017-06-21 02:14:10-0700 [Broker,client] Lost connection to master1a.golo.chromium.org:8113
2017-06-21 02:14:10-0700 [Broker,client] Stopping factory <buildslave.bot.BotFactory instance at 0x052B96C0>
2017-06-21 02:14:10-0700 [-] calling during/after trigger: <function stop_timeseries_monitoring at 0x052BC2F0>
2017-06-21 02:14:10-0700 [-] calling during/after trigger: <bound method SelectReactor._stopThreadPool of <twisted.internet.selectreactor.SelectReactor object at 0x04F3D390>>
2017-06-21 02:14:10-0700 [-] finished firing system event
2017-06-21 02:14:10-0700 [-] Main loop terminated.
2017-06-21 02:14:10-0700 [-] Server Shut Down.
2017-06-21 02:14:10-0700 [-] Server Shut Down.