update_engine: Add permissions for dlcservice user.

dlcservice needs to query Update Engine status and attempt install
operations.

BUG=chromium:906871
TEST=Check that dlcservice can send GetStatus and AttemptInstall
messages.

Change-Id: I4cfb8befbc1d49984ed8b51980a1d87f372b4c1b
Reviewed-on: https://chromium-review.googlesource.com/1342946
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Colin Howes <chowes@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/UpdateEngine.conf b/UpdateEngine.conf
index 192e6ab..9490096 100644
--- a/UpdateEngine.conf
+++ b/UpdateEngine.conf
@@ -76,4 +76,12 @@
            send_interface="org.chromium.UpdateEngineInterface"
            send_member="GetStatus"/>
   </policy>
+  <policy user="dlcservice">
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
+           send_member="GetStatus"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
+           send_member="AttemptInstall"/>
+  </policy>
 </busconfig>