Expose DBus API to enable cellular updates.

Can set UpdateEngine to either allow or prohibit updates over
cellular data network connections.

BUG=640721
TEST=none

Change-Id: I3322da7270243e0c070668c49175f34bf20eceb1
Reviewed-on: https://chromium-review.googlesource.com/441994
Commit-Ready: Niranjan Kumar <kumarniranjan@chromium.org>
Tested-by: Niranjan Kumar <kumarniranjan@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
diff --git a/dbus/update_engine/dbus-constants.h b/dbus/update_engine/dbus-constants.h
index 86c60dd..f2364b3 100644
--- a/dbus/update_engine/dbus-constants.h
+++ b/dbus/update_engine/dbus-constants.h
@@ -24,6 +24,8 @@
 const char kAttemptRollback[] = "AttemptRollback";
 const char kCanRollback[] = "CanRollback";
 const char kGetEolStatus[] = "GetEolStatus";
+const char kSetUpdateOverCellularPermission[] =
+    "SetUpdateOverCellularPermission";
 
 // Signals.
 const char kStatusUpdate[] = "StatusUpdate";