This is most certainly due to https://chromium-review.googlesource.com/c/chromium/tools/build/+/1013289

It's failing at the fetch-simplechrome-artifacts gclient hook. This isn't a simplechrome bot though (and neither is its waterfall mirror https://ci.chromium.org/buildbot/chromium.chromiumos/linux-chromeos-rel/) so it shouldn't need to do that.

I'll try to get to this soon.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/50366a857cd931901c593ba08a6e37c00a1a392e

commit 50366a857cd931901c593ba08a6e37c00a1a392e
Author: Ben Pastene <bpastene@chromium.org>
Date: Mon Apr 30 16:21:56 2018

Don't run the simplechrome gclient hook for linux-chromeos bots.

Right now the linux-chromeos bots are running the simplechrome gclient
hook. They shouldn't need to, so this stops them from doing that by
removing the "target_os=chromeos" gclient condition from their config.

This shouldn't affect the simplechrome bots.

Bug:  835955 
Change-Id: I7fe758351fccfea2b13997af5669fc4b36cc1b2d
Reviewed-on: https://chromium-review.googlesource.com/1029425
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>

[modify] https://crrev.com/50366a857cd931901c593ba08a6e37c00a1a392e/scripts/slave/recipe_modules/chromium_tests/chromium_chromiumos.py

This should be fixed now. Please reopen if it isn't.