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This class forwards user activity from ws to the
pre-servicification activity-handling mechanism, so it needs
to be used for --mus in addition to --mash. Check for
aura::Env::Mode::MUS to handle both modes.

BUG=717681
TEST=put --mus, --mash, and neither in /etc/chrome_dev.conf
     and check that user activity is reported in
     /var/log/power_manager/powerd.LATEST after hitting a
     key

Review-Url: https://codereview.chromium.org/2863623005
Cr-Commit-Position: refs/heads/master@{#469546}