Please see the logs
https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_rel_ng/builds/245315
https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_asan_rel_ng/builds/176382

I'm getting error messages like
+----------------------------------------------------------------------------------------+
| Shard 0  https://chromium-swarm.appspot.com/user/task/2f5ae213cc362610                 |
+----------------------------------------------------------------------------------------+
Failed to start Xvfb or Openbox or xcompmgr: [Errno 2] No such file or directory
+----------------------------------------------------------------------------------------+
| End of shard 0  Pending: 8.1s  Duration: 1.7s  Bot: gce-precise-ec67be82-4nhb  Exit: 1 |
+----------------------------------------------------------------------------------------+

Known bots experiencing this issue:
gce-precise-ec67be82-4nhb
gce-precise-ec67be82-fvqq
gce-precise-ec67be82-02qu
gce-precise-ec67be82-hzny