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

chrome/browser/chromeos/login/user_flow.h

Index: chrome/browser/chromeos/login/user_flow.h
diff --git a/chrome/browser/chromeos/login/user_flow.h b/chrome/browser/chromeos/login/user_flow.h
index aa5d2558b08ee03949e85acc05304e76a8114cf3..30e41cd90daac907eccff7301d3c4dcd1551bad4 100644
--- a/chrome/browser/chromeos/login/user_flow.h
+++ b/chrome/browser/chromeos/login/user_flow.h
@@ -9,6 +9,7 @@
 #include "chrome/browser/profiles/profile.h"
 #include "chromeos/login/auth/auth_status_consumer.h"
 #include "components/user_manager/user.h"
+#include "components/user_manager/user_id.h"
 
 namespace chromeos {
 
@@ -69,7 +70,7 @@ class DefaultUserFlow : public UserFlow {
 // UserFlow stub for non-regular flows.
 class ExtendedUserFlow : public UserFlow {
  public:
-  explicit ExtendedUserFlow(const std::string& user_id);
+  explicit ExtendedUserFlow(const user_manager::UserID& user_id);
   ~ExtendedUserFlow() override;
 
   void AppendAdditionalCommandLineSwitches() override;
@@ -80,12 +81,12 @@ class ExtendedUserFlow : public UserFlow {
  protected:
   // Subclasses can call this method to unregister flow in the next event.
   virtual void UnregisterFlowSoon();
-  std::string user_id() {
+  user_manager::UserID user_id() {
     return user_id_;
   }
 
  private:
-  std::string user_id_;
+  user_manager::UserID user_id_;
 };
 
 }  // namespace chromeos