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

chromeos/login/auth/user_context.h

Index: chromeos/login/auth/user_context.h
diff --git a/chromeos/login/auth/user_context.h b/chromeos/login/auth/user_context.h
index 727df7f484649b5759be9fdac81eb0bd310f0d45..9b880b7140cdc4a01848c885cab794b74cae798b 100644
--- a/chromeos/login/auth/user_context.h
+++ b/chromeos/login/auth/user_context.h
@@ -9,6 +9,7 @@
 
 #include "chromeos/chromeos_export.h"
 #include "chromeos/login/auth/key.h"
+#include "components/user_manager/user_id.h"
 #include "components/user_manager/user_type.h"
 
 namespace chromeos {
@@ -35,15 +36,14 @@ class CHROMEOS_EXPORT UserContext {
 
   UserContext();
   UserContext(const UserContext& other);
-  explicit UserContext(const std::string& user_id);
-  UserContext(user_manager::UserType user_type, const std::string& user_id);
+  explicit UserContext(const user_manager::UserID& user_id);
+  UserContext(user_manager::UserType user_type, const user_manager::UserID& user_id);
   ~UserContext();
 
   bool operator==(const UserContext& context) const;
   bool operator!=(const UserContext& context) const;
 
-  const std::string& GetUserID() const;
-  const std::string& GetGaiaID() const;
+  const user_manager::UserID& GetUserID() const;
   const Key* GetKey() const;
   Key* GetKey();
   const std::string& GetAuthCode() const;
@@ -59,8 +59,7 @@ class CHROMEOS_EXPORT UserContext {
 
   bool HasCredentials() const;
 
-  void SetUserID(const std::string& user_id);
-  void SetGaiaID(const std::string& gaia_id);
+  void SetUserID(const user_manager::UserID& user_id);
   void SetKey(const Key& key);
   void SetAuthCode(const std::string& auth_code);
   void SetRefreshToken(const std::string& refresh_token);
@@ -76,8 +75,7 @@ class CHROMEOS_EXPORT UserContext {
   void ClearSecrets();
 
  private:
-  std::string user_id_;
-  std::string gaia_id_;
+  user_manager::UserID user_id_;
   Key key_;
   std::string auth_code_;
   std::string refresh_token_;