git bisect indicates https://chromium-review.googlesource.com/c/1252632. +jamescook.

Indeed the test failure says
  ../../chrome/browser/chromeos/login/login_browsertest.cc:203: Failure
  Value of: RectContains(primary_win->bounds(), tray->GetBoundsInScreen())
    Actual: false (outer_rect (0,0 1280x800) does not contain inner_rect (1168,744 112x60))
  Expected: true

This checks the primary root window (primary display) bounds with the tray bounds, which could be affected by this CL.

I guess we can simply skip this check on Mash / SingleProcessMash.

Feel free to change the expectations or filter it out under mash. You could also hide the button in the test like this hack I did:
https://cs.chromium.org/chromium/src/ash/shelf/shelf_view_unittest.cc?q=shelf_view_unit&sq=package:chromium&l=274

Or just assign to me and I'll take care of it.

(I'm in meetings all afternoon.)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7130b4a658f92c6e33aab775715f5abb8df96aff

commit 7130b4a658f92c6e33aab775715f5abb8df96aff
Author: Jun Mukai <mukai@chromium.org>
Date: Fri Oct 05 22:20:18 2018

Do not check tray bounds in LoginTests with Mash

This expectation fails due to the button which is showing with
Mash. This is basically for developers, so it's okay to skip
the checks.

BUG= 892730 
TEST=interactive_ui_tests

Change-Id: Ibebe65891bdbb1d9539e89f42114c39435ff2737
Reviewed-on: https://chromium-review.googlesource.com/c/1265895
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597340}
[modify] https://crrev.com/7130b4a658f92c6e33aab775715f5abb8df96aff/chrome/browser/chromeos/login/login_browsertest.cc