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

ash/system/tray/system_tray_delegate.h

 // 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.
 
 #ifndef ASH_SYSTEM_TRAY_SYSTEM_TRAY_DELEGATE_H_
 #define ASH_SYSTEM_TRAY_SYSTEM_TRAY_DELEGATE_H_
 
 #include <string>
 #include <vector>
 
 #include "ash/ash_export.h"
 #include "ash/system/user/login_status.h"
 #include "ash/volume_control_delegate.h"
 #include "base/callback_forward.h"
 #include "base/files/file_path.h"
 #include "base/i18n/time_formatting.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string16.h"
 #include "ui/gfx/image/image_skia.h"
 
 namespace base {
 class TimeDelta;
 class TimeTicks;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace ash {
 
 class CustodianInfoTrayObserver;
 class ShutdownPolicyObserver;
 
 struct ASH_EXPORT NetworkIconInfo {
   NetworkIconInfo();
   ~NetworkIconInfo();
 
   bool highlight() const { return connected || connecting; }
@@ skipping 274 matching lines @@
   // The active user has been changed. This will be called when the UI is ready
   // to be switched to the new user.
   // Note: This will happen after SessionStateObserver::ActiveUserChanged fires.
   virtual void ActiveUserWasChanged();
 
   // Returns true when the Search key is configured to be treated as Caps Lock.
   virtual bool IsSearchKeyMappedToCapsLock();
 
   // Returns accounts delegate for given user. May return nullptr.
   virtual tray::UserAccountsDelegate* GetUserAccountsDelegate(
-      const std::string& user_id);
+      const user_manager::UserID& user_id);
 
   // Adding observers that are notified when supervised info is being changed.
   virtual void AddCustodianInfoTrayObserver(
       CustodianInfoTrayObserver* observer);
 
   virtual void RemoveCustodianInfoTrayObserver(
       CustodianInfoTrayObserver* observer);
 
   // Adds an observer whose |OnShutdownPolicyChanged| function is called when
   // the |DeviceRebootOnShutdown| policy changes. If this policy is set to
   // true, a device cannot be shut down anymore but only rebooted.
   virtual void AddShutdownPolicyObserver(ShutdownPolicyObserver* observer);
 
   virtual void RemoveShutdownPolicyObserver(ShutdownPolicyObserver* observer);
 
   // Determines whether the device is automatically rebooted when shut down as
   // specified by the device policy |DeviceRebootOnShutdown|. This function
   // asynchronously calls |callback| once a trusted policy becomes available.
   virtual void ShouldRebootOnShutdown(const RebootOnShutdownCallback& callback);
 
   // Returns VPNDelegate. May return nullptr.
   virtual VPNDelegate* GetVPNDelegate() const;
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_TRAY_SYSTEM_TRAY_DELEGATE_H_