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.cc b/dbus_service.cc
index 08b953d..447a900 100644
--- a/dbus_service.cc
+++ b/dbus_service.cc
@@ -107,6 +107,15 @@
   return TRUE;
 }
 
+gboolean update_engine_service_set_track(UpdateEngineService* self,
+                                         gchar* track,
+                                         GError **error) {
+  if (track) {
+    LOG(INFO) << "TODO: Setting track to: " << track;
+  }
+  return TRUE;
+}
+
 gboolean update_engine_service_emit_status_update(
     UpdateEngineService* self,
     gint64 last_checked_time,