We should use UserID object to identify users instead of username.

chrome/browser/chromeos/policy/device_local_account_external_data_service.h

 // Copyright 2013 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 CHROME_BROWSER_CHROMEOS_POLICY_DEVICE_LOCAL_ACCOUNT_EXTERNAL_DATA_SERVICE_H_
 #define CHROME_BROWSER_CHROMEOS_POLICY_DEVICE_LOCAL_ACCOUNT_EXTERNAL_DATA_SERVICE_H_
 
 #include <map>
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/chromeos/policy/device_local_account_external_data_manager.h"
 #include "chrome/browser/chromeos/policy/device_local_account_policy_service.h"
 #include "components/policy/core/common/cloud/resource_cache.h"
 
 namespace base {
 class SequencedTaskRunner;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace policy {
 
 class CloudPolicyStore;
 
 // Provides DeviceLocalAccountExternalDataManagers for all device-local
 // accounts. This class owns the |resource_cache_| that the managers share.
 class DeviceLocalAccountExternalDataService
     : public DeviceLocalAccountPolicyService::Observer {
  public:
   DeviceLocalAccountExternalDataService(
       DeviceLocalAccountPolicyService* parent,
       scoped_refptr<base::SequencedTaskRunner> backend_task_runner,
       scoped_refptr<base::SequencedTaskRunner> io_task_runner);
   ~DeviceLocalAccountExternalDataService() override;
 
   // DeviceLocalAccountPolicyService::Observer:
-  void OnPolicyUpdated(const std::string& user_id) override;
+  void OnPolicyUpdated(const user_manager::UserID& user_id) override;
   void OnDeviceLocalAccountsChanged() override;
 
   scoped_refptr<DeviceLocalAccountExternalDataManager>
       GetExternalDataManager(const std::string& account_id,
                              CloudPolicyStore* policy_store);
 
  private:
   typedef std::map<std::string,
                    scoped_refptr<DeviceLocalAccountExternalDataManager> >
       ExternalDataManagerMap;
 
   DeviceLocalAccountPolicyService* parent_;
   scoped_refptr<base::SequencedTaskRunner> backend_task_runner_;
   scoped_refptr<base::SequencedTaskRunner> io_task_runner_;
   scoped_ptr<ResourceCache> resource_cache_;
 
   ExternalDataManagerMap external_data_managers_;
 
   DISALLOW_COPY_AND_ASSIGN(DeviceLocalAccountExternalDataService);
 };
 
 }  // namespace policy
 
 #endif  // CHROME_BROWSER_CHROMEOS_POLICY_DEVICE_LOCAL_ACCOUNT_EXTERNAL_DATA_SERVICE_H_