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

chrome/browser/chromeos/login/auth/chrome_login_performer.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_AUTH_CHROME_LOGIN_PERFORMER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_AUTH_CHROME_LOGIN_PERFORMER_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/chromeos/policy/wildcard_login_checker.h"
 #include "chromeos/login/auth/auth_status_consumer.h"
 #include "chromeos/login/auth/authenticator.h"
 #include "chromeos/login/auth/extended_authenticator.h"
 #include "chromeos/login/auth/login_performer.h"
 #include "chromeos/login/auth/online_attempt_host.h"
 #include "chromeos/login/auth/user_context.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "google_apis/gaia/google_service_auth_error.h"
 
 namespace policy {
 class WildcardLoginChecker;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 // This class implements chrome-specific elements of Login Performer.
 
 class ChromeLoginPerformer : public LoginPerformer {
  public:
   explicit ChromeLoginPerformer(Delegate* delegate);
   ~ChromeLoginPerformer() override;
 
  protected:
   bool RunTrustedCheck(const base::Closure& callback) override;
   void DidRunTrustedCheck(const base::Closure& callback);
-  bool IsUserWhitelisted(const std::string& user_id,
+  bool IsUserWhitelisted(const user_manager::UserID& user_id,
                          bool* wildcard_match) override;
 
-  void RunOnlineWhitelistCheck(const std::string& user_id,
+  void RunOnlineWhitelistCheck(const user_manager::UserID& user_id,
                                bool wildcard_match,
                                const std::string& refresh_token,
                                const base::Closure& success_callback,
                                const base::Closure& failure_callback) override;
   bool AreSupervisedUsersAllowed() override;
 
   bool UseExtendedAuthenticatorForSupervisedUser(
       const UserContext& user_context) override;
 
   UserContext TransformSupervisedKey(const UserContext& context) override;
 
-  void SetupSupervisedUserFlow(const std::string& user_id) override;
+  void SetupSupervisedUserFlow(const user_manager::UserID& user_id) override;
 
-  void SetupEasyUnlockUserFlow(const std::string& user_id) override;
+  void SetupEasyUnlockUserFlow(const user_manager::UserID& user_id) override;
 
   scoped_refptr<Authenticator> CreateAuthenticator() override;
-  bool CheckPolicyForUser(const std::string& user_id) override;
+  bool CheckPolicyForUser(const user_manager::UserID& user_id) override;
   content::BrowserContext* GetSigninContext() override;
   net::URLRequestContextGetter* GetSigninRequestContext() override;
 
  private:
   void OnlineWildcardLoginCheckCompleted(
       const base::Closure& success_callback,
       const base::Closure& failure_callback,
       policy::WildcardLoginChecker::Result result);
 
   // Used to verify logins that matched wildcard on the login whitelist.
   scoped_ptr<policy::WildcardLoginChecker> wildcard_login_checker_;
   base::WeakPtrFactory<ChromeLoginPerformer> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeLoginPerformer);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_AUTH_CHROME_LOGIN_PERFORMER_H_