Toni, any chance this could be related to your CL https://chromium-review.googlesource.com/c/chromium/src/+/662403? Looking at the blamelist, this is the only CL that even looks remotely related...

Since this is not a 100% failure, more like 80% flakiness, I've looked at 

https://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/45440
https://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/45439
https://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/45438

for potential culprits as well, but the CL mentioned in comment #1 still seems to be the most likely one.

I agree that CL looks like the most probable culprit.

going to speculatively revert. FYI you can test for this flake locally on a Chrome OS linux build with: 

./browser_tests --brave-new-test-launcher --test-launcher-bot-mode --mus --ozone-platform=headless --override-use-software-gl-for-tests --test-launcher-filter-file=../../testing/buildbot/filters/mus.browser_tests.filter

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/536465432924b78d74cc4e5f4bada9ff8616b83a

commit 536465432924b78d74cc4e5f4bada9ff8616b83a
Author: Toni Barzic <tbarzic@google.com>
Date: Mon Oct 23 20:01:32 2017

Fix flakiness in supervised user password change tests

This fixes a race in SupervisedUserTestBase::StartFlowLoginAsManager.
StartFlowLoginAsManager used to wait for login profile preparation and
then force the user's OAUTH2 token status for the user. Though, during
profile creation, UserSessionManager would attempt to restore the
previous session and start async OAuth2 token fetch. If the token
fetch completed (with failure) after the test set user's oauth2 token,
supervised creation flow would end up in an error state, and cause
test failures.

This fixes SupervisedUserTestBase to inject fake oauth2 token service
into the login profile, and sets the fake refresh token for the test
user (so the oauth2 token state is set to valid during the profile
initialization).

BUG= 772387 

Change-Id: I00e30dafa372b705a429fb7070c386ccfb0985ee
Reviewed-on: https://chromium-review.googlesource.com/732183
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510886}
[modify] https://crrev.com/536465432924b78d74cc4e5f4bada9ff8616b83a/chrome/browser/chromeos/login/supervised/supervised_user_test_base.cc