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

chrome/browser/chromeos/app_mode/kiosk_app_manager.cc

Index: chrome/browser/chromeos/app_mode/kiosk_app_manager.cc
diff --git a/chrome/browser/chromeos/app_mode/kiosk_app_manager.cc b/chrome/browser/chromeos/app_mode/kiosk_app_manager.cc
index a37df89e7ee7638bc03f773960b6e5dadaf6f6b4..6ac759eb41e55edcfa8aa11bfeb3c5b8c342f016 100644
--- a/chrome/browser/chromeos/app_mode/kiosk_app_manager.cc
+++ b/chrome/browser/chromeos/app_mode/kiosk_app_manager.cc
@@ -38,6 +38,7 @@
 #include "chromeos/dbus/dbus_thread_manager.h"
 #include "chromeos/settings/cros_settings_names.h"
 #include "components/ownership/owner_key_util.h"
+#include "components/user_manager/user_id.h"
 #include "components/user_manager/user_manager.h"
 #include "content/public/browser/browser_thread.h"
 #include "extensions/common/extension_urls.h"
@@ -57,22 +58,22 @@ std::string GenerateKioskAppAccountId(const std::string& app_id) {
   return app_id + '@' + kKioskAppAccountDomain;
 }
 
-void ScheduleDelayedCryptohomeRemoval(const std::string& user_id,
+void ScheduleDelayedCryptohomeRemoval(const user_manager::UserID& user_id,
                                       const std::string& app_id) {
   PrefService* local_state = g_browser_process->local_state();
   DictionaryPrefUpdate dict_update(local_state, kKioskUsersToRemove);
-  dict_update->SetStringWithoutPathExpansion(user_id, app_id);
+  dict_update->SetStringWithoutPathExpansion(user_id.GetUserEmail(), app_id);
   local_state->CommitPendingWrite();
 }
 
-void CancelDelayedCryptohomeRemoval(const std::string& user_id) {
+void CancelDelayedCryptohomeRemoval(const user_manager::UserID& user_id) {
   PrefService* local_state = g_browser_process->local_state();
   DictionaryPrefUpdate dict_update(local_state, kKioskUsersToRemove);
-  dict_update->RemoveWithoutPathExpansion(user_id, NULL);
+  dict_update->RemoveWithoutPathExpansion(user_id.GetUserEmail(), NULL);
   local_state->CommitPendingWrite();
 }
 
-void OnRemoveAppCryptohomeComplete(const std::string& user_id,
+void OnRemoveAppCryptohomeComplete(const user_manager::UserID& user_id,
                                    const std::string& app,
                                    const base::Closure& callback,
                                    bool success,
@@ -98,7 +99,7 @@ void PerformDelayedCryptohomeRemovals(bool service_is_available) {
   const base::DictionaryValue* dict =
       local_state->GetDictionary(kKioskUsersToRemove);
   for (base::DictionaryValue::Iterator it(*dict); !it.IsAtEnd(); it.Advance()) {
-    std::string user_id = it.key();
+    const user_manager::UserID user_id(std::string() /* gaia_id */, it.key());

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:46]

FromUserEmail()

     std::string app_id;
     it.value().GetAsString(&app_id);
     VLOG(1) << "Removing obsolete crypthome for " << app_id;
@@ -175,7 +176,8 @@ KioskAppManager::App::App(
       was_auto_launched_with_zero_delay(auto_launched_with_zero_delay) {
 }
 
-KioskAppManager::App::App() : is_loading(false),
+KioskAppManager::App::App() : user_id(std::string(), std::string()),

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:46]

EmptyUserId() ? 

+                              is_loading(false),
                               was_auto_launched_with_zero_delay(false) {}
 
 KioskAppManager::App::~App() {}
@@ -609,7 +611,7 @@ void KioskAppManager::UpdateAppData() {
   const user_manager::User* active_user =
       user_manager::UserManager::Get()->GetActiveUser();
   if (active_user) {
-    std::string active_user_id = active_user->GetUserID();
+    user_manager::UserID active_user_id = active_user->GetUserID();

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:46]

auto?

     for (const auto& it : old_apps) {
       if (it.second->user_id() == active_user_id) {
         VLOG(1) << "Currently running kiosk app removed from policy, exiting";