AU: Provide a D-Bus API for changing the update track.

The method is SetTrack(string). It doesn't do anything right now but hopefully
this CL will enable work on libcros.

BUG=8104
TEST=unit tests; tested on device through update_engine_client.

Change-Id: I6941fe3d04165d85c4612e3a01939d02c02014f9

Review URL: http://codereview.chromium.org/4094001
diff --git a/dbus_service.h b/dbus_service.h
index 8c92995..b802fae 100644
--- a/dbus_service.h
+++ b/dbus_service.h
@@ -62,6 +62,10 @@
 gboolean update_engine_service_reboot_if_needed(UpdateEngineService* self,
                                                 GError **error);
 
+gboolean update_engine_service_set_track(UpdateEngineService* self,
+                                         gchar* track,
+                                         GError **error);
+
 gboolean update_engine_service_emit_status_update(
     UpdateEngineService* self,
     gint64 last_checked_time,