AU: Implement getting of tracks through GetTrack.

Add SetTrack and GetTrack to UpdateEngine.xml as well.

BUG=chromium-os:8104
TEST=tested on device with update_engine_client

Change-Id: I10ef2552e9280524b3a8d5f232d104a81a114e06

Review URL: http://codereview.chromium.org/4181001
diff --git a/dbus_service.cc b/dbus_service.cc
index 5a46b51..a27fd18 100644
--- a/dbus_service.cc
+++ b/dbus_service.cc
@@ -99,6 +99,15 @@
   return TRUE;
 }
 
+gboolean update_engine_service_get_track(UpdateEngineService* self,
+                                         gchar** track,
+                                         GError **error) {
+  string track_str =
+      chromeos_update_engine::OmahaRequestDeviceParams::GetDeviceTrack();
+  *track = strdup(track_str.c_str());
+  return TRUE;
+}
+
 gboolean update_engine_service_reboot_if_needed(UpdateEngineService* self,
                                                 GError **error) {
   if (!self->update_attempter_->RebootIfNeeded()) {