Add 'cros-disks' user to 'cdrom' and 'cdrw' group.

cros-disks now runs as the 'cros-disks' user and needs to be in the
'cdrom' and 'cdrw' group for accessing and mounting CD-ROM, CD-RW and
DVD media.

BUG=chromium:760860
CQ-DEPEND=CL:644235
TEST=Verify that Files.app can mount CD/DVD media via cros-disks.
TEST=Run updated security_AccountsBaseline (CL:644235).

Change-Id: I0453fa2b86dd7682fe0891a220703c0cc1594a08
Reviewed-on: https://chromium-review.googlesource.com/644776
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
diff --git a/profiles/base/accounts/group/cdrom b/profiles/base/accounts/group/cdrom
index 22e7cea..2bb0114 100644
--- a/profiles/base/accounts/group/cdrom
+++ b/profiles/base/accounts/group/cdrom
@@ -1,3 +1,3 @@
 group:cdrom
 gid:19
-users:
+users:cros-disks
diff --git a/profiles/base/accounts/group/cdrw b/profiles/base/accounts/group/cdrw
index 52f4b04..2679172 100644
--- a/profiles/base/accounts/group/cdrw
+++ b/profiles/base/accounts/group/cdrw
@@ -1,3 +1,3 @@
 group:cdrw
 gid:80
-users:
+users:cros-disks