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/update_engine.xml b/update_engine.xml
index 88e6871..73bc92e 100644
--- a/update_engine.xml
+++ b/update_engine.xml
@@ -20,6 +20,9 @@
     </method>
     <method name="RebootIfNeeded">
     </method>
+    <method name="SetTrack">
+      <arg type="s" name="track" />
+    </method>
     <signal name="StatusUpdate">
       <arg type="x" name="last_checked_time" />
       <arg type="d" name="progress" />