Add shill as member of modem group.

Add shill to the modem group, as cellular devices in /dev have
root:modem 0660 perms. This may be relevant to the chromium:864474, not
sure.

CQ-DEPEND=CL:1231356
BUG=b:112006434, chromium:864474
TEST=added shill to modem group, rebooted, tested cellular connectivity

Change-Id: I050938e58f52d42f134d71b9f6b6e84d724df8c3
Reviewed-on: https://chromium-review.googlesource.com/1231276
Commit-Ready: Micah Morton <mortonm@chromium.org>
Tested-by: Micah Morton <mortonm@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
diff --git a/profiles/base/accounts/group/modem b/profiles/base/accounts/group/modem
index cfd1f0b..f960f59 100644
--- a/profiles/base/accounts/group/modem
+++ b/profiles/base/accounts/group/modem
@@ -1,3 +1,3 @@
 group:modem
 gid:241
-users:
+users:shill