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

chrome/browser/ui/ash/session_state_delegate_chromeos.h

 // Copyright (c) 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_UI_ASH_SESSION_STATE_DELEGATE_CHROMEOS_H_
 #define CHROME_BROWSER_UI_ASH_SESSION_STATE_DELEGATE_CHROMEOS_H_
 
 #include "ash/session/session_state_delegate.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
@@ skipping 31 matching lines @@
   bool ShouldLockScreenBeforeSuspending() const override;
   void LockScreen() override;
   void UnlockScreen() override;
   bool IsUserSessionBlocked() const override;
   SessionState GetSessionState() const override;
   const user_manager::UserInfo* GetUserInfo(
       ash::MultiProfileIndex index) const override;
   const user_manager::UserInfo* GetUserInfo(
       content::BrowserContext* context) const override;
   bool ShouldShowAvatar(aura::Window* window) const override;
-  void SwitchActiveUser(const std::string& user_id) override;
+  void SwitchActiveUser(const user_manager::UserID& user_id) override;
   void CycleActiveUser(CycleUser cycle_user) override;
   bool IsMultiProfileAllowedByPrimaryUserPolicy() const override;
   void AddSessionStateObserver(ash::SessionStateObserver* observer) override;
   void RemoveSessionStateObserver(ash::SessionStateObserver* observer) override;
 
   // chromeos::LoginState::Observer overrides.
   void LoggedInStateChanged() override;
 
   // user_manager::UserManager::UserSessionStateObserver:
   void ActiveUserChanged(const user_manager::User* active_user) override;
   void UserAddedToSession(const user_manager::User* added_user) override;
 
   // chromeos::UserAddingScreen::Observer:
   void OnUserAddingStarted() override;
   void OnUserAddingFinished() override;
 
  private:
   // Sets session state to |new_state|.
   // If |force| is true then |new_state| is set even if existing session
   // state is the same (used for explicit initialization).
   void SetSessionState(SessionState new_state, bool force);
 
   // Notify observers about session state change.
   void NotifySessionStateChanged();
 
   // Switches to a new user. This call might show a dialog asking the user if he
   // wants to stop desktop casting before switching.
-  void TryToSwitchUser(const std::string& user_id);
+  void TryToSwitchUser(const user_manager::UserID& user_id);
 
   // List of observers is only used on Chrome OS for now.
   ObserverList<ash::SessionStateObserver> session_state_observer_list_;
 
   // Session state (e.g. login screen vs. user session).
   SessionState session_state_;
 
   DISALLOW_COPY_AND_ASSIGN(SessionStateDelegateChromeos);
 };
 
 #endif  // CHROME_BROWSER_UI_ASH_SESSION_STATE_DELEGATE_CHROMEOS_H_