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

chrome/browser/chromeos/login/session/stub_login_session_manager_delegate.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_SESSION_STUB_LOGIN_SESSION_MANAGER_DELEGATE_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SESSION_STUB_LOGIN_SESSION_MANAGER_DELEGATE_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/chromeos/login/session/restore_after_crash_session_manager_delegate.h"
 
 class Profile;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class StubLoginSessionManagerDelegate
     : public RestoreAfterCrashSessionManagerDelegate {
  public:
   StubLoginSessionManagerDelegate(Profile* profile,
-                                  const std::string& login_user_id);
+                                  const user_manager::UserID& login_user_id);
   ~StubLoginSessionManagerDelegate() override;
 
  private:
   // session_manager::SessionManagerDelegate implementation:
   void Start() override;
 
   DISALLOW_COPY_AND_ASSIGN(StubLoginSessionManagerDelegate);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SESSION_STUB_LOGIN_SESSION_MANAGER_DELEGATE_H_