shill: vpn: Add vpn user to tun group

To make a VPN client able to clone /dev/net/tun to create a tunnel
interface.

BUG=b:177984585
BUG=b:177876632
TEST=Verified that a binary running as vpn:vpn can create tunnel
  interface.

Cq-Depend: chromium:2717252
Change-Id: I714f2addb61f8ad46f868622a7de45dea3f6ff8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2705371
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Taoyu Li <taoyl@chromium.org>
Tested-by: Jie Jiang <jiejiang@chromium.org>
Commit-Queue: Jie Jiang <jiejiang@chromium.org>
diff --git a/profiles/base/accounts/group/tun b/profiles/base/accounts/group/tun
index 8d97b6c..84cfa4a 100644
--- a/profiles/base/accounts/group/tun
+++ b/profiles/base/accounts/group/tun
@@ -1,4 +1,4 @@
 # Members of this group have write access to /dev/net/tun.
 group:tun
 gid:413
-users:crosvm,shill,wpan
+users:crosvm,shill,vpn,wpan