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

ash/session/session_state_delegate.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 ASH_SESSION_SESSION_STATE_DELEGATE_H_
 #define ASH_SESSION_SESSION_STATE_DELEGATE_H_
 
 #include <string>
 #include <vector>
 
 #include "ash/ash_export.h"
 #include "base/strings/string16.h"
 
 namespace aura {
 class Window;
 }  // namespace aura
 
 namespace content {
 class BrowserContext;
 }
 
 namespace gfx {
 class ImageSkia;
 }  // namespace gfx
 
 namespace user_manager {
 class UserInfo;
+class UserID;
 }  // namespace user_manager
 
 namespace ash {
 
 class SessionStateObserver;
 
 // The index for the multi-profile item to use. The list is always LRU sorted
 // So that the index #0 is the currently active user.
 typedef int MultiProfileIndex;
 
@@ skipping 99 matching lines @@
 
   // Gets the avatar image for the user associated with the |context|.
   virtual const user_manager::UserInfo* GetUserInfo(
       content::BrowserContext* context) const = 0;
 
   // Whether or not the window's title should show the avatar.
   virtual bool ShouldShowAvatar(aura::Window* window) const = 0;
 
   // Switches to another active user with |user_id|
   // (if that user has already signed in).
-  virtual void SwitchActiveUser(const std::string& user_id) = 0;
+  virtual void SwitchActiveUser(const user_manager::UserID& user_id) = 0;
 
   // Switches the active user to the next or previous user, with the same
   // ordering as GetLoggedInUsers.
   virtual void CycleActiveUser(CycleUser cycle_user) = 0;
 
   // Returns true if primary user policy does not forbid multiple signin.
   virtual bool IsMultiProfileAllowedByPrimaryUserPolicy() const = 0;
 
   // Adds or removes sessions state observer.
   virtual void AddSessionStateObserver(SessionStateObserver* observer) = 0;
   virtual void RemoveSessionStateObserver(SessionStateObserver* observer) = 0;
 
   bool IsInSecondaryLoginScreen() const;
 };
 
 }  // namespace ash
 
 #endif  // ASH_SESSION_SESSION_STATE_DELEGATE_H_