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

chrome/browser/ui/ash/session_state_delegate_chromeos.h

Index: chrome/browser/ui/ash/session_state_delegate_chromeos.h
diff --git a/chrome/browser/ui/ash/session_state_delegate_chromeos.h b/chrome/browser/ui/ash/session_state_delegate_chromeos.h
index 2b261483a317579def625b087f75595839e936e6..45f9d776382fddfe6f79f725d90b6bd2564cfb62 100644
--- a/chrome/browser/ui/ash/session_state_delegate_chromeos.h
+++ b/chrome/browser/ui/ash/session_state_delegate_chromeos.h
@@ -49,7 +49,7 @@ class SessionStateDelegateChromeos
   const user_manager::UserInfo* GetUserInfo(
       content::BrowserContext* context) const override;
   bool ShouldShowAvatar(aura::Window* window) const override;
-  void SwitchActiveUser(const std::string& user_id) override;
+  void SwitchActiveUser(const user_manager::UserID& user_id) override;
   void CycleActiveUser(CycleUser cycle_user) override;
   bool IsMultiProfileAllowedByPrimaryUserPolicy() const override;
   void AddSessionStateObserver(ash::SessionStateObserver* observer) override;
@@ -77,7 +77,7 @@ class SessionStateDelegateChromeos
 
   // Switches to a new user. This call might show a dialog asking the user if he
   // wants to stop desktop casting before switching.
-  void TryToSwitchUser(const std::string& user_id);
+  void TryToSwitchUser(const user_manager::UserID& user_id);
 
   // List of observers is only used on Chrome OS for now.
   ObserverList<ash::SessionStateObserver> session_state_observer_list_;