Add 'cros-disks' user to 'avfs' group.

cros-disks now runs as the 'cros-disks' user and needs to be in the
'avfs' for accessing sub-directories under /run/avfsroot, which belongs
to the 'avfs' group, for mounting archive files via AVFS.

BUG=chromium:763225
CQ-DEPEND=CL:734297
CQ-DEPEND=CL:733957
TEST=Verify that Files.app can mount a RAR archive via cros-disks.
TEST=Run updated security_AccountsBaseline (CL:734297).

Change-Id: If6d19b6462548952c01aa2d884e85b367f8da976
Reviewed-on: https://chromium-review.googlesource.com/734313
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
(cherry picked from commit dc3b51764fb155f79a82a497e990e4848c7d9c21)
Reviewed-on: https://chromium-review.googlesource.com/738852
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/profiles/base/accounts/group/avfs b/profiles/base/accounts/group/avfs
index ed18dbc..a029984 100644
--- a/profiles/base/accounts/group/avfs
+++ b/profiles/base/accounts/group/avfs
@@ -1,3 +1,3 @@
 group:avfs
 gid:301
-users:
+users:cros-disks