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

components/user_manager/empty_user_info.h

Index: components/user_manager/empty_user_info.h
diff --git a/components/user_manager/empty_user_info.h b/components/user_manager/empty_user_info.h
index 52d677c3e2fd0ba65dc2b64aad667696a804b847..abda4f038b9b4595b04aacefbabb8f41efa507ed 100644
--- a/components/user_manager/empty_user_info.h
+++ b/components/user_manager/empty_user_info.h
@@ -25,11 +25,12 @@ class USER_MANAGER_EXPORT EmptyUserInfo : public UserInfo {
   base::string16 GetDisplayName() const override;
   base::string16 GetGivenName() const override;
   std::string GetEmail() const override;
-  std::string GetUserID() const override;
+  const UserID& GetUserID() const override;
   const gfx::ImageSkia& GetImage() const override;
 
  private:
   const gfx::ImageSkia null_image_;
+  const UserID user_id_;
 
   DISALLOW_COPY_AND_ASSIGN(EmptyUserInfo);
 };