Tips:
* You might want to add this test to the existing desktopui_MashLogin tests rather than adding a new autotest. Autotests can be very slow because they involve deploying scripts to the device each run. Unlike unit tests we often tolerate tests being bigger and doing multiple things.
* desktopui_MashLogin is in https://cs.corp.google.com/chromeos_public/src/third_party/autotest/files/client/site_tests/desktopui_MashLogin/desktopui_MashLogin.py?q=desktopui_MashLogin&sq=package:%5Echromeos_public$&l=11
* You have to have a Chrome OS checkout to work on the tests
* There's a hacky way to iterate on the test without deploying it each time, ping me for details
* My go-to people for autotest questions are achuith, ihf, xiyuan. There's also the internal chrome os infrastructure mailing list.

Test CC.

An autotest that does --mus passes on link for me. I'll try and add it bvt-perbuild to see what happens.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/6b82663bbadc80d898e4978ffd0035578ed88d8c

commit 6b82663bbadc80d898e4978ffd0035578ed88d8c
Author: kylechar <kylechar@chromium.org>
Date: Sat May 06 01:29:02 2017

autotest: Add desktopui_MusLogin to bvt-perbuild.

Add a new test desktopui_MusLogin to the informational bvt-perbuild.
This test is similar to desktopui_MashLogin except it tests "./chrome
--mus" instead of "./chrome --mash."

BUG= chromium:708737 
TEST=Ran autotest on board link.

Change-Id: I18b8da2a1f7e81584d43a3f66d78e067a8dad394
Reviewed-on: https://chromium-review.googlesource.com/497907
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
Tested-by: Kyle Charbonneau <kylechar@chromium.org>
Commit-Queue: Kyle Charbonneau <kylechar@chromium.org>

[add] https://crrev.com/6b82663bbadc80d898e4978ffd0035578ed88d8c/client/site_tests/desktopui_MusLogin/desktopui_MusLogin.py
[add] https://crrev.com/6b82663bbadc80d898e4978ffd0035578ed88d8c/client/site_tests/desktopui_MusLogin/control

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/14dfe062729c37fe23bf7d5876d3fe08361a4485

commit 14dfe062729c37fe23bf7d5876d3fe08361a4485
Author: kylechar <kylechar@chromium.org>
Date: Mon May 08 19:40:59 2017

autotest: Add desktopui_MusLogin to bvt-perbuild.

Add a new test desktopui_MusLogin to the informational bvt-perbuild.
This test is similar to desktopui_MashLogin except it tests "./chrome
--mus" instead of "./chrome --mash."

BUG= chromium:708737 
TEST=Ran autotest on board link.

Change-Id: I7f6c74b9012c8fbf36b546e03e5f508a31d133a8
Reviewed-on: https://chromium-review.googlesource.com/497967
Commit-Ready: Kyle Charbonneau <kylechar@chromium.org>
Tested-by: Kyle Charbonneau <kylechar@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>

[modify] https://crrev.com/14dfe062729c37fe23bf7d5876d3fe08361a4485/chromeos-base/autotest-chrome/autotest-chrome-9999.ebuild

Check here for the test results (may take a day or two)
https://wmatrix.googleplex.com/unfiltered?hide_missing=True&releases=tot&tests=desktopui_MusLogin

Thanks for the help ihf. I see test results, marking as fixed.