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/omaha_request_params.cc b/omaha_request_params.cc
index 03327b2..4cc5426 100644
--- a/omaha_request_params.cc
+++ b/omaha_request_params.cc
@@ -92,6 +92,13 @@
   return params.SetTrack(track);
 }
 
+string OmahaRequestDeviceParams::GetDeviceTrack() {
+  OmahaRequestDeviceParams params;
+  // Note that params.app_track is an empty string if the value in
+  // lsb-release file is invalid. See Init() for details.
+  return params.Init("", "") ? params.app_track : "";
+}
+
 string OmahaRequestDeviceParams::GetLsbValue(const string& key,
                                              const string& default_value,
                                              ValueValidator validator) const {