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

chrome/browser/profiles/profile_info_interface.h

Index: chrome/browser/profiles/profile_info_interface.h
diff --git a/chrome/browser/profiles/profile_info_interface.h b/chrome/browser/profiles/profile_info_interface.h
index aa8d11edf9e7f9d320f6d7112a04fd10586d3022..7e96c9252df5ef4f5cb756e037d4631916d6138c 100644
--- a/chrome/browser/profiles/profile_info_interface.h
+++ b/chrome/browser/profiles/profile_info_interface.h
@@ -16,6 +16,10 @@ namespace gfx {
 class Image;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 // This abstract interface is used to query the profiles backend for information
 // about the different profiles. Its sole concrete implementation is the
 // ProfileInfoCache. This interface exists largely to assist in testing.
@@ -72,7 +76,7 @@ class ProfileInfoInterface {
   // list (see ProfileListDesktop), so it won't appear in the avatar menu.
   virtual bool IsOmittedProfileAtIndex(size_t index) const = 0;
 
-  virtual std::string GetSupervisedUserIdOfProfileAtIndex(
+  virtual user_manager::UserID GetSupervisedUserIdOfProfileAtIndex(
       size_t index) const = 0;
 
   // This profile is associated with an account but has been signed-out.