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

chrome/browser/profiles/profile_manager.h

Index: chrome/browser/profiles/profile_manager.h
diff --git a/chrome/browser/profiles/profile_manager.h b/chrome/browser/profiles/profile_manager.h
index 4b5c8a881d7fcd4c2e277c249aec3c3697b91aa8..358e1957f682f583036bea584c00ea32fef155c7 100644
--- a/chrome/browser/profiles/profile_manager.h
+++ b/chrome/browser/profiles/profile_manager.h
@@ -21,6 +21,7 @@
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_shortcut_manager.h"
 #include "chrome/browser/ui/browser_list_observer.h"
+#include "components/user_manager/user_id.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 
@@ -91,7 +92,7 @@ class ProfileManager : public base::NonThreadSafe,
                           const CreateCallback& callback,
                           const base::string16& name,
                           const base::string16& icon_url,
-                          const std::string& supervised_user_id);
+                          const user_manager::UserID& supervised_user_id);
 
   // Returns true if the profile pointer is known to point to an existing
   // profile.
@@ -143,7 +144,7 @@ class ProfileManager : public base::NonThreadSafe,
       const base::string16& name,
       const base::string16& icon_url,
       const CreateCallback& callback,
-      const std::string& supervised_user_id);
+      const user_manager::UserID& supervised_user_id);
 
   // Returns the full path to be used for guest profiles.
   static base::FilePath GetGuestProfilePath();