The familiar condition where the slave did not reboot.  From twistd.log:

2017-08-10 09:12:38-0700 [-] command finished with signal None, exit code 0, elapsedTime: 3045.765000
2017-08-10 09:12:38-0700 [-] SlaveBuilder.commandComplete <buildslave.commands.shell.SlaveShellCommand instance at 0x056EFC88>
2017-08-10 09:12:38-0700 [Broker,client] Setting needs_reboot flag
2017-08-10 09:12:38-0700 [Broker,client] slave shutting down on command from master
2017-08-10 09:12:38-0700 [-] firing system event shutdown
2017-08-10 09:12:38-0700 [-] calling before trigger: <bound method MultiService.stopService of <twisted.application.service.MultiService instance at 0x054E0260>>
2017-08-10 09:12:38-0700 [-] stopping service <buildslave.bot.BuildSlave instance at 0x054E0350>
2017-08-10 09:12:38-0700 [-] stopping service <twisted.application.internet.TCPClient instance at 0x054E04E0>
2017-08-10 09:12:38-0700 [-] stopped service <twisted.application.internet.TCPClient instance at 0x054E04E0>
2017-08-10 09:12:38-0700 [-] stopping service <buildslave.bot.Bot instance at 0x054E0378>
2017-08-10 09:12:38-0700 [-] stopping service <SlaveBuilder 'Win Builder' at 91008296>
2017-08-10 09:12:38-0700 [-] stopped service <SlaveBuilder 'Win Builder' at 91008296>
2017-08-10 09:12:38-0700 [-] stopped service <buildslave.bot.Bot instance at 0x054E0378>
2017-08-10 09:12:38-0700 [-] stopped service <buildslave.bot.BuildSlave instance at 0x054E0350>
2017-08-10 09:12:38-0700 [-] calling before trigger: <bound method TimerService.stopService of <twisted.application.internet.TimerService instance at 0x055412B0>>
2017-08-10 09:12:38-0700 [-] calling during/after trigger: <bound method SelectReactor.crash of <twisted.internet.selectreactor.SelectReactor object at 0x05143E70>>
2017-08-10 09:12:38-0700 [-] calling during/after trigger: <bound method SelectReactor.disconnectAll of <twisted.internet.selectreactor.SelectReactor object at 0x05143E70>>
2017-08-10 09:12:38-0700 [Broker,client] lost remote
2017-08-10 09:12:38-0700 [Broker,client] Lost connection to master1a.golo.chromium.org:8113
2017-08-10 09:12:38-0700 [Broker,client] Stopping factory <buildslave.bot.BotFactory instance at 0x054E04B8>
2017-08-10 09:12:38-0700 [-] calling during/after trigger: <function stop_timeseries_monitoring at 0x054E1530>
2017-08-10 09:12:38-0700 [-] calling during/after trigger: <bound method SelectReactor._stopThreadPool of <twisted.internet.selectreactor.SelectReactor object at 0x05143E70>>
2017-08-10 09:12:38-0700 [-] finished firing system event
2017-08-10 09:12:38-0700 [-] Main loop terminated.
2017-08-10 09:12:38-0700 [-] Server Shut Down.
2017-08-10 09:12:38-0700 [-] Server Shut Down.