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

chrome/browser/chromeos/login/auth/chrome_login_performer.h

Index: chrome/browser/chromeos/login/auth/chrome_login_performer.h
diff --git a/chrome/browser/chromeos/login/auth/chrome_login_performer.h b/chrome/browser/chromeos/login/auth/chrome_login_performer.h
index 0920508c7fad8be440401c5302dcdccc8ece3301..ecf8da7667ae574cef63e804a11105ede91ff8a9 100644
--- a/chrome/browser/chromeos/login/auth/chrome_login_performer.h
+++ b/chrome/browser/chromeos/login/auth/chrome_login_performer.h
@@ -25,6 +25,10 @@ namespace policy {
 class WildcardLoginChecker;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 // This class implements chrome-specific elements of Login Performer.
@@ -37,10 +41,10 @@ class ChromeLoginPerformer : public LoginPerformer {
  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,
@@ -52,12 +56,12 @@ class ChromeLoginPerformer : public LoginPerformer {
 
   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;