AU: Use Omaha ID rather than MAC address in delta updater

Review URL: http://codereview.chromium.org/467051
diff --git a/omaha_request_prep_action.h b/omaha_request_prep_action.h
index ca7fd17..cb82a9f 100644
--- a/omaha_request_prep_action.h
+++ b/omaha_request_prep_action.h
@@ -49,7 +49,7 @@
 
  private:
   // Gets a machine-local ID (for now, first MAC address we find)
-  std::string GetMachineId() const;
+  bool GetMachineId(std::string* out_id) const;
 
   // Fetches the value for a given key from
   // /mnt/stateful_partition/etc/lsb-release if possible. Failing that,