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

chromeos/login/auth/login_performer.h

Index: chromeos/login/auth/login_performer.h
diff --git a/chromeos/login/auth/login_performer.h b/chromeos/login/auth/login_performer.h
index bf78330c6ed5d596a8b436252bff5658e5911e7a..c5c17a5cffc1c03fff124f3048664c404da79794 100644
--- a/chromeos/login/auth/login_performer.h
+++ b/chromeos/login/auth/login_performer.h
@@ -31,6 +31,10 @@ namespace content {
 class BrowserContext;
 }
 
+namespace user_manager {
+class UserID;
+} // namespace user_manager
+
 namespace chromeos {
 
 // This class encapsulates sign in operations.
@@ -55,9 +59,9 @@ class CHROMEOS_EXPORT LoginPerformer : public AuthStatusConsumer,
   class Delegate : public AuthStatusConsumer {
    public:
     ~Delegate() override {}
-    virtual void WhiteListCheckFailed(const std::string& email) = 0;
+    virtual void WhiteListCheckFailed(const user_manager::UserID& user_id) = 0;
     virtual void PolicyLoadFailed() = 0;
-    virtual void OnOnlineChecked(const std::string& email, bool success) = 0;
+    virtual void OnOnlineChecked(const user_manager::UserID& user_id, bool success) = 0;
   };
 
   LoginPerformer(scoped_refptr<base::TaskRunner> task_runner,
@@ -81,7 +85,7 @@ class CHROMEOS_EXPORT LoginPerformer : public AuthStatusConsumer,
   void LoginAsPublicSession(const UserContext& user_context);
 
   // Performs a login into the kiosk mode account with |app_user_id|.
-  void LoginAsKioskAccount(const std::string& app_user_id,
+  void LoginAsKioskAccount(const user_manager::UserID& app_user_id,
                            bool use_guest_mount);
 
   // AuthStatusConsumer implementation:
@@ -119,7 +123,7 @@ class CHROMEOS_EXPORT LoginPerformer : public AuthStatusConsumer,
 
  protected:
   // Implements OnlineAttemptHost::Delegate.
-  void OnChecked(const std::string& user_id, bool success) override;
+  void OnChecked(const user_manager::UserID& user_id, bool success) override;
 
   // Platform-dependant methods to be implemented by concrete class.
 
@@ -131,14 +135,14 @@ class CHROMEOS_EXPORT LoginPerformer : public AuthStatusConsumer,
   // Check if user is allowed to sign in on device. |wildcard_match| will
   // contain additional information whether this user is explicitly listed or
   // not (may be relevant for extension-based sign-in).
-  virtual bool IsUserWhitelisted(const std::string& user_id,
+  virtual bool IsUserWhitelisted(const user_manager::UserID& user_id,
                                  bool* wildcard_match) = 0;
 
   // This method should run addional online check if user can sign in on device.
   // Either |success_callback| or |failure_callback| should be called upon this
   // check.
   virtual void RunOnlineWhitelistCheck(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       bool wildcard_match,
       const std::string& refresh_token,
       const base::Closure& success_callback,
@@ -157,14 +161,14 @@ class CHROMEOS_EXPORT LoginPerformer : public AuthStatusConsumer,
   virtual UserContext TransformSupervisedKey(const UserContext& context) = 0;
 
   // Set up sign-in flow for supervised user.
-  virtual void SetupSupervisedUserFlow(const std::string& user_id) = 0;
+  virtual void SetupSupervisedUserFlow(const user_manager::UserID& user_id) = 0;
 
   // Set up sign-in flow for Easy Unlock.
-  virtual void SetupEasyUnlockUserFlow(const std::string& user_id) = 0;
+  virtual void SetupEasyUnlockUserFlow(const user_manager::UserID& user_id) = 0;
 
   // Run policy check for |user_id|. If something is wrong, delegate's
   // PolicyLoadFailed is called.
-  virtual bool CheckPolicyForUser(const std::string& user_id) = 0;
+  virtual bool CheckPolicyForUser(const user_manager::UserID& user_id) = 0;
 
   // Look up browser context to use during signin.
   virtual content::BrowserContext* GetSigninContext() = 0;