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This makes it so aura::Env is configured for mus if --mus is specified
on the command line.

BUG=776514
TEST=test only change

Change-Id: Iaeb80b571e274f5bae9af7688d6b55b79e43b3d2
Reviewed-on: https://chromium-review.googlesource.com/823568Reviewed-by: Elliot Glaysher <erg@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523824}