I tried a prospective fix ( https://chromium-review.googlesource.com/c/chromium/src/+/807887 ) and it crashed after a few seconds of running/interacting:

msw@msw-linux:/work/chrome-git/src$ out/Default/chrome --mus --user-data-dir=/tmp/msw_cros_1
[140535:140535:1205/091939.672669:ERROR:zygote_host_impl_linux.cc(111)] SUID sandbox binary is missing. Won't be able to adjust OOM scores.
[140535:140559:1205/091940.841328:ERROR:logging_chrome.cc(210)] Unable to create symlink /tmp/msw_cros_1/test-user/chrome_debug.log pointing at /tmp/msw_cros_1/test-user/chrome_debug_20171205-091940: File exists (17)
[140535:140535:1205/091940.857604:ERROR:display_manager.cc(1735)] Could not find display:36029742295586560
[140535:140577:1205/091940.928597:ERROR:binder_registry.h(89)] Failed to locate a binder for interface: ui::mojom::VideoDetector
[140535:140535:1205/091940.932654:ERROR:easy_resize_window_targeter.cc(29)] Not implemented reached in gfx::Insets wm::(anonymous namespace)::InsetsWithOnlyNegativeValues(const gfx::Insets &)
[140535:140535:1205/091941.040283:ERROR:input_method_manager_impl.cc(1031)] IMEEngine for "fgoepimhcoialccpbmpnnblemnepkkao" is not registered
ATTENTION: default value of option force_s3tc_enable overridden by environment.
[140572:140572:1205/091941.571806:ERROR:sandbox_linux.cc(375)] InitializeSandbox() called with multiple threads in process gpu-process.
[140535:140535:1205/091944.494313:ERROR:tab_manager_delegate_chromeos.cc(82)] Set OOM score error: 
[140535:140535:1205/091950.372102:ERROR:tab_manager_delegate_chromeos.cc(82)] Set OOM score error: 
[xcb] Unknown sequence number while processing queue
[xcb] Most likely this is a multi-threaded client and XInitThreads has not been called
[xcb] Aborting, sorry about that.
-git/src/out/Default/chrome --mus --user-data-dir=/tmp/msw_cros_1: ../../src/xcb_io.c:274: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.
Received signal 6
#0 0x7f7a99d5c8cc base::debug::StackTrace::StackTrace()
#1 0x7f7a99d5c3c1 base::debug::(anonymous namespace)::StackDumpSignalHandler()
#2 0x7f7a99ee3330 <unknown>
#3 0x7f7a8d4b4c37 gsignal
#4 0x7f7a8d4b8028 abort
#5 0x7f7a8d4adbf6 <unknown>
#6 0x7f7a8d4adca2 __assert_fail
#7 0x7f7a8cc95c69 <unknown>
#8 0x7f7a8cc95cfc <unknown>
#9 0x7f7a8cc95fcd _XEventsQueued
#10 0x7f7a8cc8812d XPending
#11 0x7f7a8c3727c6 ui::X11EventSource::DispatchXEvents()
#12 0x7f7a99d93aa3 base::MessagePumpLibevent::OnLibeventNotification()
#13 0x7f7a99e5b76d event_base_loop
#14 0x7f7a99d93de4 base::MessagePumpLibevent::Run()
#15 0x7f7a99d90bc9 base::MessageLoop::Run()
#16 0x7f7a99dc5399 base::RunLoop::Run()
#17 0x7f7a99e05787 base::Thread::Run()
#18 0x7f7a99e05d3c base::Thread::ThreadMain()
#19 0x7f7a99dfcadf base::(anonymous namespace)::ThreadFunc()
#20 0x7f7a99edb184 start_thread
#21 0x7f7a8d57bffd clone
  r8: fefefefefefefeff  r9: ff092d63646b6860 r10: 0000000000000008 r11: 0000000000000202
 r12: 00007f7a8cd05390 r13: 00007f7a8cd05696 r14: 00007f7a6fd38528 r15: 00007f7a99ebbae8
  di: 00000000000224f7  si: 0000000000022521  bp: 00007f7a8d602018  bx: 00007f7a70f28000
  dx: 0000000000000006  ax: 0000000000000000  cx: 00007f7a8d4b4c37  sp: 00007f7a6fd382a8
  ip: 00007f7a8d4b4c37 efl: 0000000000000202 cgf: 0000000000000033 erf: 0000000000000000
 trp: 0000000000000000 msk: 0000000000000000 cr2: 0000000000000000
[end of stack trace]
Calling _exit(1). Core file will not be generated.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6ae6cf7e61175b7bfcf1e5e3aab238785dc3d0f0

commit 6ae6cf7e61175b7bfcf1e5e3aab238785dc3d0f0
Author: Sadrul Habib Chowdhury <sadrul@chromium.org>
Date: Tue Dec 05 23:49:16 2017

mus/aura: Wait for the real AcceleratedWidget.

When mus does not host viz, aura handles the real AcceleratedWidget. So
in such cases, WindowTreeHostMus needs to wait for the widget to be
avaiable, which is still created by mus and sent to aura over mojom.

BUG= 786453 ,  791619 

Change-Id: I335365bfe4793c8fafb8d08eaa45c09c50e17a59
Reviewed-on: https://chromium-review.googlesource.com/807887
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521903}
[modify] https://crrev.com/6ae6cf7e61175b7bfcf1e5e3aab238785dc3d0f0/ash/shell_port_mus.cc
[modify] https://crrev.com/6ae6cf7e61175b7bfcf1e5e3aab238785dc3d0f0/ui/aura/mus/window_tree_client.cc
[modify] https://crrev.com/6ae6cf7e61175b7bfcf1e5e3aab238785dc3d0f0/ui/aura/mus/window_tree_host_mus.cc
[modify] https://crrev.com/6ae6cf7e61175b7bfcf1e5e3aab238785dc3d0f0/ui/aura/mus/window_tree_host_mus_init_params.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bbe89b730b652156b1e621a87c686f75179f86a5

commit bbe89b730b652156b1e621a87c686f75179f86a5
Author: Sadrul Habib Chowdhury <sadrul@chromium.org>
Date: Wed Dec 06 01:12:02 2017

ozone-x11: Always initialize in multi-thread mode.

Like single-process mode, chrome uses x11 from multiple threads in
--mus mode too (the window-server thread, and the viz thread). So
always initialize ozone-x11 in multi-thread mode. It would be possible
to introduce a new flag in InitParams for this mode, but considering
this is only used for development purposes on developer workstation,
just init multi-thread mode unconditionally.

BUG= 786453 ,  791619 

Change-Id: I930fc325ef1a52f531a419f660d31f55af03d1c6
Reviewed-on: https://chromium-review.googlesource.com/809943
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521933}
[modify] https://crrev.com/bbe89b730b652156b1e621a87c686f75179f86a5/ui/ozone/platform/x11/ozone_platform_x11.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/904d1dfc4a5bad0842a09c41a659bb18a0bb6bc4

commit 904d1dfc4a5bad0842a09c41a659bb18a0bb6bc4
Author: Kunihiko Sakamoto <ksakamoto@chromium.org>
Date: Wed Dec 06 05:33:49 2017

Revert "mus/aura: Wait for the real AcceleratedWidget."

This reverts commit 6ae6cf7e61175b7bfcf1e5e3aab238785dc3d0f0.

Reason for revert: Speculative revert for ash_unittests-mus failure on chromium.chromiumos/linux-chromeos-rel, started here:
https://uberchromegw.corp.google.com/i/chromium.chromiumos/builders/linux-chromeos-rel/builds/3239
This is the only mus related CL in the regression range.


Original change's description:
> mus/aura: Wait for the real AcceleratedWidget.
> 
> When mus does not host viz, aura handles the real AcceleratedWidget. So
> in such cases, WindowTreeHostMus needs to wait for the widget to be
> avaiable, which is still created by mus and sent to aura over mojom.
> 
> BUG= 786453 ,  791619 
> 
> Change-Id: I335365bfe4793c8fafb8d08eaa45c09c50e17a59
> Reviewed-on: https://chromium-review.googlesource.com/807887
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#521903}

TBR=sadrul@chromium.org,sky@chromium.org,msw@chromium.org

Change-Id: I9b15d1ad8fc36e6a4b4864e1f7bfd6df2b3745a5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug:  786453 ,  791619 
Reviewed-on: https://chromium-review.googlesource.com/810404
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522019}
[modify] https://crrev.com/904d1dfc4a5bad0842a09c41a659bb18a0bb6bc4/ash/shell_port_mus.cc
[modify] https://crrev.com/904d1dfc4a5bad0842a09c41a659bb18a0bb6bc4/ui/aura/mus/window_tree_client.cc
[modify] https://crrev.com/904d1dfc4a5bad0842a09c41a659bb18a0bb6bc4/ui/aura/mus/window_tree_host_mus.cc
[modify] https://crrev.com/904d1dfc4a5bad0842a09c41a659bb18a0bb6bc4/ui/aura/mus/window_tree_host_mus_init_params.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b34782ec012125060545fb68b9fc2d70870996be

commit b34782ec012125060545fb68b9fc2d70870996be
Author: Sadrul Habib Chowdhury <sadrul@chromium.org>
Date: Fri Dec 08 17:05:43 2017

mus/aura: Wait for the real AcceleratedWidget.

When mus does not host viz, aura handles the real AcceleratedWidget. So
in such cases, WindowTreeHostMus needs to wait for the widget to be
avaiable, which is still created by mus and sent to aura over mojom.

BUG= 786453 ,  791619 

Change-Id: I4e206211ffabdc6f0ae09066f7a3ec720659322b
Reviewed-on: https://chromium-review.googlesource.com/809242
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522800}
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ash/shell_port_mus.cc
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ash/test/ash_test_base.cc
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ash/test/ash_test_helper.cc
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ash/test/ash_test_helper.h
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ui/aura/BUILD.gn
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ui/aura/mus/window_tree_client.cc
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ui/aura/mus/window_tree_host_mus.cc
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ui/aura/mus/window_tree_host_mus_init_params.h
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ui/aura/test/mus/test_window_tree.cc
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ui/aura/test/mus/test_window_tree.h
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ui/aura/test/mus/test_window_tree_client_setup.cc
[modify] https://crrev.com/b34782ec012125060545fb68b9fc2d70870996be/ui/aura/test/mus/test_window_tree_client_setup.h

Migrating Proj-Mustash-Mus to components Internals>Services>WindowService and Internals>Services>Ash

I'm closing this out assuming it's no longer an issue. If I'm wrong, please reopen.