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

chrome/browser/feedback/show_feedback_page.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <string>
 
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/extensions/api/feedback_private/feedback_private_api.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_manager.h"
@@ skipping 48 matching lines @@
   }
 
   // We do not want to launch on an OTR profile.
   profile = profile->GetOriginalProfile();
   DCHECK(profile);
 
 #if defined(OS_CHROMEOS)
   // Obtains the display profile ID on which the Feedback window should show.
   chrome::MultiUserWindowManager* const window_manager =
       chrome::MultiUserWindowManager::GetInstance();
-  const std::string display_profile_id =
+  const user_manager::UserID display_profile_id =
       window_manager && browser
           ? window_manager->GetUserPresentingWindow(
                 browser->window()->GetNativeWindow())
-          : "";
+          : user_manager::UserID(std::string(), std::string());

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:44]

EmptyUserID

   profile = display_profile_id.empty()
                 ? profile
                 : multi_user_util::GetProfileFromUserID(display_profile_id);
 #endif
 
   extensions::FeedbackPrivateAPI* api =
       extensions::FeedbackPrivateAPI::GetFactoryInstance()->Get(profile);
 
   api->RequestFeedback(description_template,
                        category_tag,
                        page_url);
 }
 
 }  // namespace chrome