In this config the browser process contains chrome and ash. Mus is in
a separate process. This patch is just the plumbing for the switch and
detection, it doesn't actually work yet.

BUG=704985
TEST=none
R=rockot@chromium.org, tsepez@chromium.org

Review-Url: https://codereview.chromium.org/2775873002
Cr-Commit-Position: refs/heads/master@{#459599}