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.h b/omaha_request_params.h
index d7d2493..b8e2ee5 100644
--- a/omaha_request_params.h
+++ b/omaha_request_params.h
@@ -80,6 +80,9 @@
   bool SetTrack(const std::string& track);
   static bool SetDeviceTrack(const std::string& track);
 
+  // Returns the release track. On error, returns an empty string.
+  static std::string GetDeviceTrack();
+
   // For unit-tests.
   void set_root(const std::string& root) { root_ = root; }