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

chrome/browser/chromeos/policy/user_cloud_policy_store_chromeos.h

 // Copyright (c) 2012 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_USER_CLOUD_POLICY_STORE_CHROMEOS_H_
 #define CHROME_BROWSER_CHROMEOS_POLICY_USER_CLOUD_POLICY_STORE_CHROMEOS_H_
 
 #include <string>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "chromeos/dbus/dbus_method_call_status.h"
 #include "components/policy/core/common/cloud/cloud_policy_validator.h"
 #include "components/policy/core/common/cloud/user_cloud_policy_store_base.h"
+#include "components/user_manager/user_id.h"
 
 namespace base {
 class SequencedTaskRunner;
 }
 
 namespace chromeos {
 class CryptohomeClient;
 class SessionManagerClient;
 }
 
 namespace policy {
 
 class LegacyPolicyCacheLoader;
 
 // Implements a cloud policy store backed by the Chrome OS' session_manager,
 // which takes care of persisting policy to disk and is accessed via DBus calls
 // through SessionManagerClient.
 //
 // Additionally, this class drives legacy UserPolicyTokenCache and
 // UserPolicyDiskCache instances, migrating policy from these to session_manager
 // storage on the fly.
 class UserCloudPolicyStoreChromeOS : public UserCloudPolicyStoreBase {
  public:
   UserCloudPolicyStoreChromeOS(
       chromeos::CryptohomeClient* cryptohome_client,
       chromeos::SessionManagerClient* session_manager_client,
       scoped_refptr<base::SequencedTaskRunner> background_task_runner,
-      const std::string& username,
+      const user_manager::UserID& user_id,
       const base::FilePath& user_policy_key_dir,
       const base::FilePath& legacy_token_cache_file,
       const base::FilePath& legacy_policy_cache_file);
   ~UserCloudPolicyStoreChromeOS() override;
 
   // CloudPolicyStore:
   void Store(const enterprise_management::PolicyFetchResponse& policy) override;
   void Load() override;
 
   // Loads the policy synchronously on the current thread.
@@ skipping 59 matching lines @@
   // Callback for getting the sanitized username from |cryptohome_client_|.
   void OnGetSanitizedUsername(const base::Closure& callback,
                               chromeos::DBusMethodCallStatus call_status,
                               const std::string& sanitized_username);
 
   scoped_ptr<UserCloudPolicyValidator> CreateValidatorForLoad(
       scoped_ptr<enterprise_management::PolicyFetchResponse> policy);
 
   chromeos::CryptohomeClient* cryptohome_client_;
   chromeos::SessionManagerClient* session_manager_client_;
-  const std::string username_;
+  const user_manager::UserID user_id_;
   base::FilePath user_policy_key_dir_;
 
   // TODO(mnissler): Remove all the legacy policy support members below after
   // the number of pre-M20 clients drops back to zero.
   base::FilePath legacy_cache_dir_;
   scoped_ptr<LegacyPolicyCacheLoader> legacy_loader_;
   bool legacy_caches_loaded_;
 
   bool policy_key_loaded_;
   base::FilePath policy_key_path_;
   std::string policy_key_;
 
   base::WeakPtrFactory<UserCloudPolicyStoreChromeOS> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(UserCloudPolicyStoreChromeOS);
 };
 
 }  // namespace policy
 
 #endif  // CHROME_BROWSER_CHROMEOS_POLICY_USER_CLOUD_POLICY_STORE_CHROMEOS_H_