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

chrome/browser/chromeos/login/users/multi_profile_user_controller.h

 // Copyright 2014 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_LOGIN_USERS_MULTI_PROFILE_USER_CONTROLLER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_USERS_MULTI_PROFILE_USER_CONTROLLER_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_vector.h"
 
 class PrefChangeRegistrar;
 class PrefRegistrySimple;
 class PrefService;
 class Profile;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace user_prefs {
 class PrefRegistrySyncable;
 }
 
 namespace chromeos {
 
 class MultiProfileUserControllerDelegate;
 class UserManager;
 
 // MultiProfileUserController decides whether a user is allowed to be in a
@@ skipping 27 matching lines @@
     NOT_ALLOWED_POLICY_FORBIDS
   };
 
   MultiProfileUserController(MultiProfileUserControllerDelegate* delegate,
                              PrefService* local_state);
   ~MultiProfileUserController();
 
   static void RegisterPrefs(PrefRegistrySimple* registry);
   static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
 
-  // Returns the cached policy value for |user_email|.
-  std::string GetCachedValue(const std::string& user_email) const;
+  // Returns the cached policy value for |user_id|.
+  std::string GetCachedValue(const user_manager::UserID& user_id) const;
 
   // Returns primary user policy (only ALLOW,
   // NOT_ALLOWED_PRIMARY_POLICY_CERT_TAINTED,
   // NOT_ALLOWED_PRIMARY_USER_POLICY_FORBIDS)
   static UserAllowedInSessionReason GetPrimaryUserPolicy();
 
   // Returns true if user allowed to be in the current session. If |reason| not
   // null stores UserAllowedInSessionReason enum that describes actual reason.
-  bool IsUserAllowedInSession(const std::string& user_email,
+  bool IsUserAllowedInSession(const user_manager::UserID& user_id,
                               UserAllowedInSessionReason* reason) const;
 
   // Starts to observe the multiprofile user behavior pref of the given profile.
   void StartObserving(Profile* user_profile);
 
   // Removes the cached values for the given user.
-  void RemoveCachedValues(const std::string& user_email);
+  void RemoveCachedValues(const user_manager::UserID& user_id);
 
   // Possible behavior values.
   static const char kBehaviorUnrestricted[];
   static const char kBehaviorPrimaryOnly[];
   static const char kBehaviorNotAllowed[];
   static const char kBehaviorOwnerPrimaryOnly[];
 
  private:
   friend class MultiProfileUserControllerTest;
 
   // Sets the cached policy value.
-  void SetCachedValue(const std::string& user_email,
+  void SetCachedValue(const user_manager::UserID& user_id,
                       const std::string& behavior);
 
   // Checks if all users are allowed in the current session.
   void CheckSessionUsers();
 
   // Invoked when user behavior pref value changes.
   void OnUserPrefChanged(Profile* profile);
 
   MultiProfileUserControllerDelegate* delegate_;  // Not owned.
   PrefService* local_state_;  // Not owned.
   ScopedVector<PrefChangeRegistrar> pref_watchers_;
 
   DISALLOW_COPY_AND_ASSIGN(MultiProfileUserController);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_USERS_MULTI_PROFILE_USER_CONTROLLER_H_