cros_workon: disallow running as root, it doesn't work anyway

BUG=chromium-os:30384
TEST=install it, run as root, see it fail

Change-Id: Id852af26e2985a86f02212e6c35aeff3324e8b88
Reviewed-on: https://gerrit.chromium.org/gerrit/21857
Commit-Ready: Zdenek Behan <zbehan@chromium.org>
Tested-by: Zdenek Behan <zbehan@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/host/cros_workon b/host/cros_workon
index 682198f..0b0863c 100755
--- a/host/cros_workon
+++ b/host/cros_workon
@@ -12,6 +12,8 @@
 
 . /usr/lib/crosutils/common.sh || exit 1
 
+assert_not_root_user
+
 # Script must be run inside the chroot
 get_default_board