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

chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.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 "chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.h"
 
 #include "ash/desktop_background/user_wallpaper_delegate.h"
 #include "ash/root_window_controller.h"
 #include "ash/shelf/shelf_layout_manager.h"
 #include "ash/shelf/shelf_widget.h"
@@ skipping 84 matching lines @@
   if (itr != window_list->end()) {
     window_list->erase(itr);
     window_list->push_back(active_window);
   }
 }
 
 }  // namespace
 
 UserSwitchAnimatorChromeOS::UserSwitchAnimatorChromeOS(
     MultiUserWindowManagerChromeOS* owner,
-    const std::string& new_user_id,
+    const user_manager::UserID& new_user_id,
     int animation_speed_ms)
     : owner_(owner),
       new_user_id_(new_user_id),
       animation_speed_ms_(animation_speed_ms),
       animation_step_(ANIMATION_STEP_HIDE_OLD_USER),
       screen_cover_(GetScreenCover(NULL)),
       windows_by_user_id_() {
   BuildUserToWindowsListMap();
   AdvanceUserTransitionAnimation();
 
@@ skipping 71 matching lines @@
       ash::Shell::GetInstance()->user_wallpaper_delegate();
   if (animation_step == ANIMATION_STEP_HIDE_OLD_USER) {
     // Set the wallpaper cross dissolve animation duration to our complete
     // animation cycle for a fade in and fade out.
     int duration =
         NO_USER_COVERS_SCREEN == screen_cover_ ? (2 * animation_speed_ms_) : 0;
     wallpaper_delegate->SetAnimationDurationOverride(
         std::max(duration, kMinimalAnimationTimeMS));
     if (screen_cover_ != NEW_USER_COVERS_SCREEN) {
       chromeos::WallpaperManager::Get()->SetUserWallpaperNow(new_user_id_);
-      wallpaper_user_id_ =
+      wallpaper_user_email_ =
           (NO_USER_COVERS_SCREEN == screen_cover_ ? "->" : "") +
-          new_user_id_;
+          new_user_id_.GetUserEmail();
     }
   } else if (animation_step == ANIMATION_STEP_FINALIZE) {
     // Revert the wallpaper cross dissolve animation duration back to the
     // default.
     if (screen_cover_ == NEW_USER_COVERS_SCREEN)
       chromeos::WallpaperManager::Get()->SetUserWallpaperNow(new_user_id_);
 
     // Coming here the wallpaper user id is the final result. No matter how we
     // got here.
-    wallpaper_user_id_ = new_user_id_;
+    wallpaper_user_email_ = new_user_id_.GetUserEmail();
     wallpaper_delegate->SetAnimationDurationOverride(0);
   }
 }
 
 void UserSwitchAnimatorChromeOS::TransitionUserShelf(
     AnimationStep animation_step) {
   ChromeLauncherController* chrome_launcher_controller =
       ChromeLauncherController::instance();
   // The shelf animation duration override.
   int duration_override = animation_speed_ms_;
@@ skipping 223 matching lines @@
         if (itr != window_to_entry_map.end()) {
           windows_by_user_id_[itr->second->show_for_user()].push_back(
               child_window);
         }
       }
     }
   }
 }
 
 }  // namespace chrome