upstart: Allow cras real time priority.
upstart: Allow cras real time priority.

Let cras run with real time priority.  The total time real time
processes consume is limited to 800ms per second.

BUG=chromiuim:213118
TEST=check rtprio of cras with ps.

Change-Id: I2db56d3c07a7c7d8c16f81458d9625cabac7aa90
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/51491
Reviewed-by: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>
Reviewed-by: Richard Barnette <9e7d66387742bf40b0d2548ef2b93e7c522ca0eb@chromium.org>
Reviewed-by: Wajahat Siddiqui <3c3ea4adfdf19decee174766aef6add34b32b7f0@gmail.com>
Reviewed-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>

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