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

src/platform/update_engine/omaha_request_prep_action.h

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROMEOS_PLATFORM_UPDATE_ENGINE_OMAHA_RESQUEST_PREP_ACTION_H__
 #define CHROMEOS_PLATFORM_UPDATE_ENGINE_OMAHA_RESQUEST_PREP_ACTION_H__
 
 #include <string>
 #include "update_engine/action.h"
 #include "update_engine/update_check_action.h"
@@ skipping 31 matching lines @@
   static std::string StaticType() { return "OmahaRequestPrepAction"; }
   std::string Type() const { return StaticType(); }
 
   // For unit-tests.
   void set_root(const std::string& root) {
     root_ = root;
   }
 
  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,
   // it looks for the key in /etc/lsb-release .
   std::string GetLsbValue(const std::string& key) const;
 
   // Gets the machine type (e.g. "i686")
   std::string GetMachineType() const;
 
   // Set to true if this should set up the Update Check Action to do
   // a full update
   bool force_full_update_;
 
   // When reading files, prepend root_ to the paths. Useful for testing.
   std::string root_;
 
   DISALLOW_COPY_AND_ASSIGN(OmahaRequestPrepAction);
 };
 
 }  // namespace chromeos_update_engine
 
 #endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_OMAHA_RESQUEST_PREP_ACTION_H__