I've seen a few examples of similar failures recently. A shard of webkit_layout_tests times out without any output (no stdout or files), making the result invalid.

Build: https://ci.chromium.org/buildbot/tryserver.chromium.mac/mac_chromium_rel_ng/663530
Job: https://chromium-swarm.appspot.com/task?id=3bffb6ceada58d10&refresh=10&show_raw=1

This is most common on Mac jobs, but I'm not sure if it's limited to Mac only.