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

chrome/browser/chromeos/accessibility/accessibility_manager.h

 // Copyright (c) 2013 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_ACCESSIBILITY_ACCESSIBILITY_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_ACCESSIBILITY_ACCESSIBILITY_MANAGER_H_
 
 #include <set>
 
 #include "ash/session/session_state_observer.h"
@@ skipping 10 matching lines @@
 #include "extensions/browser/event_router.h"
 #include "extensions/browser/extension_system.h"
 #include "ui/base/ime/chromeos/input_method_manager.h"
 #include "ui/chromeos/accessibility_types.h"
 
 namespace content {
 class RenderViewHost;
 }
 class Profile;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 enum AccessibilityNotificationType {
   ACCESSIBILITY_MANAGER_SHUTDOWN,
   ACCESSIBILITY_TOGGLE_HIGH_CONTRAST_MODE,
   ACCESSIBILITY_TOGGLE_LARGE_CURSOR,
   ACCESSIBILITY_TOGGLE_SCREEN_MAGNIFIER,
   ACCESSIBILITY_TOGGLE_SPOKEN_FEEDBACK,
   ACCESSIBILITY_TOGGLE_VIRTUAL_KEYBOARD,
   ACCESSIBILITY_BRAILLE_DISPLAY_CONNECTION_STATE_CHANGED
@@ skipping 114 matching lines @@
   // Enables or disables the virtual keyboard.
   void EnableVirtualKeyboard(bool enabled);
   // Returns true if the virtual keyboard is enabled, otherwise false.
   bool IsVirtualKeyboardEnabled();
 
   // Returns true if a braille display is connected to the system, otherwise
   // false.
   bool IsBrailleDisplayConnected() const;
 
   // SessionStateObserver overrides:
-  void ActiveUserChanged(const std::string& user_id) override;
+  void ActiveUserChanged(const user_manager::UserID& user_id) override;
 
   void SetProfileForTest(Profile* profile);
 
   static void SetBrailleControllerForTest(
       extensions::api::braille_display_private::BrailleController* controller);
 
   // Enables/disables system sounds.
   void EnableSystemSounds(bool system_sounds_enabled);
 
   // Initiates play of shutdown sound and returns it's duration.
@@ skipping 110 matching lines @@
   bool braille_ime_current_;
 
   base::WeakPtrFactory<AccessibilityManager> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(AccessibilityManager);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_ACCESSIBILITY_ACCESSIBILITY_MANAGER_H_