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

chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.h

Index: chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.h
diff --git a/chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.h b/chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.h
index 3afc8eb136144de5c36fce7c093ed2b172fd299e..f73ab83cf2277158636fdf328a9b9784c3d454fd 100644
--- a/chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.h
+++ b/chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.h
@@ -9,6 +9,7 @@
 #include <set>
 #include <string>
 
+#include "components/user_manager/user_id.h"
 #include "ui/message_center/notification_blocker.h"
 
 // A notification blocker for per-profile stream switching. Owned and controlled
@@ -18,11 +19,11 @@ class MultiUserNotificationBlockerChromeOS
  public:
   MultiUserNotificationBlockerChromeOS(
       message_center::MessageCenter* message_center,
-      const std::string& initial_user_id);
+      const user_manager::UserID& initial_user_id);
   ~MultiUserNotificationBlockerChromeOS() override;
 
   // Called by MultiUserWindowManager when the active user has changed.
-  void ActiveUserChanged(const std::string& user_id);
+  void ActiveUserChanged(const user_manager::UserID& user_id);
 
   // message_center::NotificationBlocker overrides:
   bool ShouldShowNotification(
@@ -34,8 +35,8 @@ class MultiUserNotificationBlockerChromeOS
   // Returns true if this blocker is actively working.
   bool IsActive() const;
 
-  std::string active_user_id_;
-  std::map<std::string, bool> quiet_modes_;
+  user_manager::UserID active_user_id_;
+  std::map<user_manager::UserID, bool> quiet_modes_;
 
   DISALLOW_COPY_AND_ASSIGN(MultiUserNotificationBlockerChromeOS);
 };