https://listnr.corp.google.com/report/74695215759
Happened only once.  Device was in my backpack.

From Justin:

The end of eventlog shows the system trying to reboot about every 30 seconds. It was doing this for at least 15 minutes.

229 | 2017-09-28 22:48:20 | System boot | 484
230 | 2017-09-28 22:48:20 | Power Fail
231 | 2017-09-28 22:48:20 | SUS Power Fail
232 | 2017-09-28 22:48:20 | System Reset
233 | 2017-09-28 22:48:20 | ACPI Wake | S5
234 | 2017-09-28 22:48:20 | Wake Source | Power Button | 0
235 | 2017-09-28 22:48:20 | Chrome OS Developer Mode
236 | 2017-09-28 22:48:29 | Kernel Event | Clean Shutdown
237 | 2017-09-28 22:48:29 | ACPI Enter | S5
238 | 2017-09-28 22:48:51 | System boot | 485
239 | 2017-09-28 22:48:51 | Power Fail
240 | 2017-09-28 22:48:51 | SUS Power Fail
241 | 2017-09-28 22:48:51 | System Reset
242 | 2017-09-28 22:48:51 | ACPI Wake | S5
243 | 2017-09-28 22:48:51 | Wake Source | Power Button | 0
244 | 2017-09-28 22:48:51 | Chrome OS Developer Mode
245 | 2017-09-28 22:49:00 | Kernel Event | Clean Shutdown
246 | 2017-09-28 22:49:00 | ACPI Enter | S5
247 | 2017-09-28 22:49:21 | System boot | 486
248 | 2017-09-28 22:49:21 | Power Fail
249 | 2017-09-28 22:49:21 | SUS Power Fail
250 | 2017-09-28 22:49:21 | System Reset
251 | 2017-09-28 22:49:21 | ACPI Wake | S5
252 | 2017-09-28 22:49:21 | Wake Source | Power Button | 0
253 | 2017-09-28 22:49:21 | Chrome OS Developer Mode
254 | 2017-09-28 22:49:31 | Kernel Event | Clean Shutdown
255 | 2017-09-28 22:49:31 | ACPI Enter | S5
256 | 2017-09-28 22:49:52 | System boot | 487
257 | 2017-09-28 22:49:52 | Power Fail
258 | 2017-09-28 22:49:52 | SUS Power Fail
259 | 2017-09-28 22:49:52 | System Reset
260 | 2017-09-28 22:49:52 | ACPI Wake | S5
261 | 2017-09-28 22:49:52 | Wake Source | Power Button | 0
262 | 2017-09-28 22:49:52 | Chrome OS Developer Mode
263 | 2017-09-28 22:49:52 | Log area cleared | 1031
264 | 2017-09-28 22:50:02 | Kernel Event | Clean Shutdown
265 | 2017-09-28 22:50:02 | ACPI Enter | S5
266 | 2017-09-28 22:50:14 | System boot | 488
267 | 2017-09-28 22:50:14 | SUS Power Fail
268 | 2017-09-28 22:50:14 | ACPI Wake | S5
269 | 2017-09-28 22:50:14 | Wake Source | Power Button | 0
270 | 2017-09-28 22:50:14 | EC Event | Lid Open
271 | 2017-09-28 22:50:14 | EC Event | Power Button
272 | 2017-09-28 22:50:14 | Chrome OS Developer Mode

From syslog:
2017-09-28T22:48:59.813184-07:00 NOTICE pre-shutdown[3316]: Shutting down for halt: state-transition
2017-09-28T22:49:30.706360-07:00 NOTICE pre-shutdown[3302]: Shutting down for halt: state-transition
2017-09-28T22:50:01.799472-07:00 NOTICE pre-shutdown[3312]: Shutting down for halt: state-transition


...and from powerd.PREVIOUS:
[0928/224956:INFO:daemon.cc(311)] Tablet mode enabled at startup
[0928/224956:INFO:daemon.cc(314)] Lid closed at startup
...
[0928/224959:INFO:state_controller.cc(931)] Ready to perform lid-closed action (shutdown)
[0928/224959:INFO:display_power_setter.cc(82)] Asking Chrome to turn all displays off
[0928/225001:INFO:internal_backlight_controller.cc(682)] Setting brightness to 0 (0%) over 0 ms
[0928/225001:INFO:daemon.cc(1570)] Shutting down, reason: state-transition