I believe for this, ShellPortMus::AddVideoDetectorObserver() should do what ShellPortLocal does (i.e. get HostFrameSinkManager from aura::Env and add observer there) when !switches::IsMusHostingViz(). And that should be sufficient.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6f42bde1a82319bfe994413bf09758abb0aaffcb

commit 6f42bde1a82319bfe994413bf09758abb0aaffcb
Author: Saman Sami <samans@chromium.org>
Date: Wed Dec 13 22:29:04 2017

Make video detection work with --mus

With --mus the browser process hosts viz so we must directly talk to
HostFrameSinkManager to add a VideoDetectorObserver instead of going
through the ui service which has no access to viz whatsoever.

Tested on my workstation both --mus, --mus=viz and --mash.

Bug:  792575 
Change-Id: Ifba73334ddc667af0a2b6076c3add9ada59f3362
Reviewed-on: https://chromium-review.googlesource.com/823103
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Saman Sami <samans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523913}
[modify] https://crrev.com/6f42bde1a82319bfe994413bf09758abb0aaffcb/ash/shell_port_mus.cc