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

chrome/browser/ui/ash/multi_user/multi_user_util.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_UI_ASH_MULTI_USER_MULTI_USER_UTIL_H_
 #define CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_UTIL_H_
 
 #include <string>
 
 class Profile;
 
 namespace aura {
 class Window;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace multi_user_util {
 
 // Get the user id from a given profile.
-std::string GetUserIDFromProfile(Profile* profile);
+user_manager::UserID GetUserIDFromProfile(Profile* profile);
 
 // Get the user id from an email address.
-std::string GetUserIDFromEmail(const std::string& email);
+user_manager::UserID GetUserIDFromEmail(const std::string& email);
 
 // Get a profile for a given user id.
-Profile* GetProfileFromUserID(const std::string& user_id);
+Profile* GetProfileFromUserID(const user_manager::UserID& user_id);
 
 // Get a profile for a |window|. Returns NULL if window belongs to no profile.
 Profile* GetProfileFromWindow(aura::Window* window);
 
 // Check if the given profile is from the currently active user. Note that since
 // only Chrome OS has the concept of an active user, only Chrome OS can ever
 // return false.
 bool IsProfileFromActiveUser(Profile* profile);
 
 // Gets the current user. Note that this will only return for ChromeOS a string.
 // All other operating systems will get an empty string.
-const std::string& GetCurrentUserId();
+const user_manager::UserID& GetCurrentUserId();
 
 // Move the window to the current user's desktop.
 void MoveWindowToCurrentDesktop(aura::Window* window);
 
 }  // namespace multi_user_util
 
 #endif  // CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_UTIL_H_