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Use --mus=viz if the mus window-server is the viz-host. If --mus is
not set to viz, then mus is used, but chrome still is the viz host.

For now, always set it to 'viz', so that mus still remains the viz
host. Once some more fixes land, we can turn it off by default.

BUG=786453

Change-Id: If8f6fca0178322311a0a14b718b08f9084bda35c
Reviewed-on: https://chromium-review.googlesource.com/792276
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519840}