Add 'firewall' user for firewalld.

We can now run firewalld as a non-root user using ambient capabilities.

BUG=chromium:715678
TEST=Build image, check /etc/{passwd,group}.

Change-Id: I9a38d60c109bd8b4e06bec06c15974be1fd9cae1
Reviewed-on: https://chromium-review.googlesource.com/488301
Commit-Ready: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
diff --git a/profiles/base/accounts/group/firewall b/profiles/base/accounts/group/firewall
new file mode 100644
index 0000000..249e534
--- /dev/null
+++ b/profiles/base/accounts/group/firewall
@@ -0,0 +1,3 @@
+group:firewall
+gid:236
+users:
diff --git a/profiles/base/accounts/user/firewall b/profiles/base/accounts/user/firewall
new file mode 100644
index 0000000..c12bef7
--- /dev/null
+++ b/profiles/base/accounts/user/firewall
@@ -0,0 +1,6 @@
+user:firewall
+uid:236
+gid:236
+gecos:firewall daemon
+home:/dev/null
+shell:/bin/false