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

chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc

Index: chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc
diff --git a/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc b/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc
index 5c49d80e66d4e789c60654a1a2c836163472fc6e..e603422e3de152efa55e569218d2baa41a20abe1 100644
--- a/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc
+++ b/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.cc
@@ -102,7 +102,7 @@ void PutMruWindowLast(std::vector<aura::Window*>* window_list) {
 
 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),
@@ -194,9 +194,9 @@ void UserSwitchAnimatorChromeOS::TransitionWallpaper(
         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
@@ -206,7 +206,7 @@ void UserSwitchAnimatorChromeOS::TransitionWallpaper(
 
     // 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);
   }
 }