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

chrome/browser/chromeos/accessibility/magnification_manager.cc

 // Copyright (c) 2012 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 "chrome/browser/chromeos/accessibility/magnification_manager.h"
 
 #include <limits>
 
 #include "ash/magnifier/magnification_controller.h"
 #include "ash/magnifier/partial_magnification_controller.h"
@@ skipping 88 matching lines @@
     if (!profile_)
       return std::numeric_limits<double>::min();
 
     return profile_->GetPrefs()->GetDouble(
         prefs::kAccessibilityScreenMagnifierScale);
   }
 
   void SetProfileForTest(Profile* profile) override { SetProfile(profile); }
 
   // SessionStateObserver overrides:
-  void ActiveUserChanged(const std::string& user_id) override {
+  void ActiveUserChanged(const user_manager::UserID& user_id) override {
     SetProfile(ProfileManager::GetActiveUserProfile());
   }
 
  private:
   void SetProfile(Profile* profile) {
     pref_change_registrar_.reset();
 
     if (profile) {
       // TODO(yoshiki): Move following code to PrefHandler.
       pref_change_registrar_.reset(new PrefChangeRegistrar);
@@ skipping 168 matching lines @@
   delete g_magnification_manager;
   g_magnification_manager = NULL;
 }
 
 // static
 MagnificationManager* MagnificationManager::Get() {
   return g_magnification_manager;
 }
 
 }  // namespace chromeos