Add midis user to audio group

This allows the MIDI service to perform ioctls on /dev/snd.

BUG=chromium:701005
TEST=emerge-veyron_minnie midis
CQ-DEPEND=CL:*338552

Change-Id: Id9447dd73c36d662ca534b3a54deb06dfa8f6879
Reviewed-on: https://chromium-review.googlesource.com/456743
Commit-Ready: Prashant Malani <pmalani@google.com>
Tested-by: Prashant Malani <pmalani@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/profiles/base/accounts/group/audio b/profiles/base/accounts/group/audio
index 6b4e663..abef6f7 100644
--- a/profiles/base/accounts/group/audio
+++ b/profiles/base/accounts/group/audio
@@ -1,3 +1,3 @@
 group:audio
 gid:18
-users:cras,chronos,volume
+users:cras,chronos,volume,midis