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Freon supports a user mode console that is drm based.  So,
to transition between the user console and chrome, we will
initiate dbus messages to effect the transition.  This change
is the chromeos/dbus part.

BUG=406031
TEST=test with user mode console

Review URL: https://codereview.chromium.org/697493002

Cr-Commit-Position: refs/heads/master@{#302666}