Mike, please kick this back to me if you have too much going on. I'm assigning to you as you are likely familiar with this.

Hey Ahmed, I applied the latest patch set (#9) of your linked CL.
The test passes when run in isolation (or with all DockedMagnifierTest.*) with or without --mus.
Can you confirm that you're still experiencing a failure here?

$ ash_unittests --gtest_filter=DockedMagnifierTest.AddRemoveDisplays --mus
...
[ RUN      ] DockedMagnifierTest.AddRemoveDisplays
[160310:160310:0222/140009.228783:1477974630754:ERROR:display_manager.cc(1931)] Could not find display:102
[       OK ] DockedMagnifierTest.AddRemoveDisplays (161 ms)
[----------] 1 test from DockedMagnifierTest (161 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (161 ms total)
[  PASSED  ] 1 test.
....

I rebased again, and I can still see the same failure:

[ RUN      ] DockedMagnifierTest.AddRemoveDisplays
[185644:185644:0222/171504.904509:2100655740206:ERROR:display_manager.cc(1931)] Could not find display:102
../../ash/magnifier/docked_magnifier_controller_unittest.cc:336: Failure
Expected: (nullptr) != (viewport_widget), actual: 8-byte object <00-00 00-00 00-00 00-00> vs NULL
[185644:185651:0222/171505.141752:2100655977442:ERROR:gl_context_virtual.cc(39)] Trying to make virtual context current without decoder.
[185644:185651:0222/171505.148393:2100655984080:ERROR:gl_context_virtual.cc(39)] Trying to make virtual context current without decoder.
[  FAILED  ] DockedMagnifierTest.AddRemoveDisplays (523 ms)

This is the same failure as seen on the bot: https://build.chromium.org/deprecated/tryserver.chromium.chromiumos/builders/linux-chromeos-rel/builds/64267/steps/mus_ash_unittests%20(with%20patch)/logs/DockedMagnifierTest.AddRemoveDisplays

I rebased from #537362 to #538651, it repros with --enable-features=Mash, but not with --enable-features=Mus.

Here are my gn args:
is_component_build = true
target_os = "chromeos"
use_goma = true
use_ozone = true
ozone_auto_platforms = false
ozone_platform_headless = true
ozone_platform_x11 = true
ozone_platform = "x11"
dcheck_always_on = true
is_debug = false

And here are the bot's gn args:
dcheck_always_on = true
ffmpeg_branding = "ChromeOS"
goma_dir = "/b/c/goma_client"
is_component_build = false
is_debug = false
proprietary_codecs = true
strip_absolute_paths_from_debug_symbols = true
symbol_level = 1
target_os = "chromeos"
use_goma = true
use_vaapi = true

I might try building with the bots args to see if I can repro with mus; otherwise I'll investigate with mash.

I didn't use --enable-features, I just used ash_unittests --mus.

Here are my gn args:

symbol_level=-1
enable_profiling=true 
is_debug=true 
v8_enable_backtrace=true 
is_component_build=true 
enable_nacl=true 
use_ozone=true 
remove_webcore_debug_symbols=true 
use_goma=true 
is_chrome_branded=true 
target_os="chromeos"

See Scott's recent email (I'm guessing this also means --mus/--mash don't work on ash_unittests):
Assuming https://chromium-review.googlesource.com/c/chromium/src/+/927360 doesn't get reverted --mus and --mash are now features. What this means is --mus/--mash no longer do anything. Instead you have to use --enable-features=Mus or --enable-features=Mash. Please let me know if you run into any problems.

I rebuilt with your gn args and found that it only fails with --enable-features=Mash. It passes with --enable-features=Mus, --mus, and --mash.

Un-cc-ing me from all bugs on my final day.

Obsolete. We're removing Config::MUS.