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

chrome/browser/chromeos/login/users/user_manager_interface.h

Index: chrome/browser/chromeos/login/users/user_manager_interface.h
diff --git a/chrome/browser/chromeos/login/users/user_manager_interface.h b/chrome/browser/chromeos/login/users/user_manager_interface.h
index 4ca26eb8a517597a358a9c4c7ef675adca79f2e8..61eb33f1bc2e99f435cf00e9da3bce1cb231d247 100644
--- a/chrome/browser/chromeos/login/users/user_manager_interface.h
+++ b/chrome/browser/chromeos/login/users/user_manager_interface.h
@@ -25,14 +25,14 @@ class UserManagerInterface {
 
   virtual BootstrapManager* GetBootstrapManager() = 0;
   virtual MultiProfileUserController* GetMultiProfileUserController() = 0;
-  virtual UserImageManager* GetUserImageManager(const std::string& user_id) = 0;
+  virtual UserImageManager* GetUserImageManager(const user_manager::UserID& user_id) = 0;
   virtual SupervisedUserManager* GetSupervisedUserManager() = 0;
 
   // Method that allows to set |flow| for user identified by |user_id|.
   // Flow should be set before login attempt.
   // Takes ownership of the |flow|, |flow| will be deleted in case of login
   // failure.
-  virtual void SetUserFlow(const std::string& user_id, UserFlow* flow) = 0;
+  virtual void SetUserFlow(const user_manager::UserID& user_id, UserFlow* flow) = 0;
 
   // Return user flow for current user. Returns instance of DefaultUserFlow if
   // no flow was defined for current user, or user is not logged in.
@@ -42,10 +42,10 @@ class UserManagerInterface {
   // Return user flow for user identified by |user_id|. Returns instance of
   // DefaultUserFlow if no flow was defined for user.
   // Returned value should not be cached.
-  virtual UserFlow* GetUserFlow(const std::string& user_id) const = 0;
+  virtual UserFlow* GetUserFlow(const user_manager::UserID& user_id) const = 0;
 
   // Resets user flow for user identified by |user_id|.
-  virtual void ResetUserFlow(const std::string& user_id) = 0;
+  virtual void ResetUserFlow(const user_manager::UserID& user_id) = 0;
 
   // Returns list of users allowed for supervised user creation.
   // Returns an empty list in cases when supervised user creation or adding new