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

components/user_manager/user_info_impl.h

Index: components/user_manager/user_info_impl.h
diff --git a/components/user_manager/user_info_impl.h b/components/user_manager/user_info_impl.h
index d23ca0c34439c17e38d9936e6444dcbd1e19ad33..d3d565c27f1436a7aa1639e333370f1f49574bc4 100644
--- a/components/user_manager/user_info_impl.h
+++ b/components/user_manager/user_info_impl.h
@@ -24,11 +24,12 @@ class USER_MANAGER_EXPORT UserInfoImpl : 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:
   gfx::ImageSkia user_image_;
+  const UserID user_id_;
 
   DISALLOW_COPY_AND_ASSIGN(UserInfoImpl);
 };