shill: vpn: create 'ppp' group for /dev/ppp permission control

This allows 'vpn' user to be removed from 'shill' group.

BUG=b:177984585
TEST=build

Cq-Depend: chromium:3275815, chromium:3276212, chromium:3276380
Change-Id: I8ccebc905131e6c71416c63e9a5ecc53f6546492
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/3277675
Commit-Queue: Taoyu Li <taoyl@chromium.org>
Tested-by: Taoyu Li <taoyl@chromium.org>
Reviewed-by: Jie Jiang <jiejiang@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
diff --git a/profiles/base/accounts/group/ppp b/profiles/base/accounts/group/ppp
new file mode 100644
index 0000000..4174f0f
--- /dev/null
+++ b/profiles/base/accounts/group/ppp
@@ -0,0 +1,5 @@
+# Members of this group have access to /dev/ppp udev file, i.e. can create
+# ppp interfaces.
+group:ppp
+gid:424
+users:shill,vpn
diff --git a/profiles/base/accounts/group/shill b/profiles/base/accounts/group/shill
index f0b9000..173be1e 100644
--- a/profiles/base/accounts/group/shill
+++ b/profiles/base/accounts/group/shill
@@ -1,3 +1,3 @@
 group:shill
 gid:20104
-users:shill,vpn
+users:shill