Bug 883296  seems a bit related. Adding it here for visibility.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a612d699fc5fa699bab6e8557f16372af4a5190d

commit a612d699fc5fa699bab6e8557f16372af4a5190d
Author: James Cook <jamescook@chromium.org>
Date: Thu Oct 18 20:29:00 2018

chromeos: Fix DeviceLocalAccountTest under SingleProcessMash

Window close events are sent over mojo between browser and ash. The
test needs to wait for in-flight messages to complete before making
assertions about the state of ash.

Also fix a problem where the window delete message wasn't being sent
by WindowTreeClient.

Bug:  877118 
Test: DeviceLocalAccountTest.LastWindowClosedLogoutReminder
Change-Id: I43145c8140462428a7db050489593038745c8f94
Reviewed-on: https://chromium-review.googlesource.com/c/1284059
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600886}
[modify] https://crrev.com/a612d699fc5fa699bab6e8557f16372af4a5190d/chrome/browser/chromeos/policy/device_local_account_browsertest.cc
[modify] https://crrev.com/a612d699fc5fa699bab6e8557f16372af4a5190d/testing/buildbot/filters/chromeos.single_process_mash.browser_tests.filter
[modify] https://crrev.com/a612d699fc5fa699bab6e8557f16372af4a5190d/ui/aura/mus/window_tree_client.cc

There are bugs filed for individual failures, so I'm closing this one out.