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

components/user_manager/user_info.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 COMPONENTS_USER_MANAGER_USER_INFO_H_
 #define COMPONENTS_USER_MANAGER_USER_INFO_H_
 
 #include <string>
 
 #include "base/strings/string16.h"
@@ skipping 18 matching lines @@
   // Gets the given name of the user.
   virtual base::string16 GetGivenName() const = 0;
 
   // Gets the display email address for the user.
   // The display email address might contains some periods in the email name
   // as well as capitalized letters. For example: "Foo.Bar@mock.com".
   virtual std::string GetEmail() const = 0;
 
   // Gets the user id (sanitized email address) for the user.
   // The function would return something like "foobar@mock.com".
-  virtual UserID GetUserID() const = 0;
+  virtual const UserID& GetUserID() const = 0;
 
   // Gets the avatar image for the user.
   virtual const gfx::ImageSkia& GetImage() const = 0;
 };
 
 }  // namespace user_manager
 
 #endif  // COMPONENTS_USER_MANAGER_USER_INFO_H_