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Run mus_demo_unittests on the X11 Chrome OS trybot. This test just
checks that mojo:mus_demo starts and draws a frame. When tcmalloc
problems are fixed this should run on Ozone Chrome OS trybot (ideally
with Ozone X11).

BUG=611473

Review-Url: https://codereview.chromium.org/2151093003
Cr-Commit-Position: refs/heads/master@{#419226}