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

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

Index: chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.h
diff --git a/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.h b/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.h
index e3e627a586418b50b543bb7079b470fe70c92bfc..89ac8f2750af84b78e15e486af42f2076e334a6d 100644
--- a/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.h
+++ b/chrome/browser/ui/ash/multi_user/user_switch_animator_chromeos.h
@@ -10,6 +10,7 @@
 
 #include "base/memory/scoped_ptr.h"
 #include "base/timer/timer.h"
+#include "components/user_manager/user_id.h"
 #include "ui/aura/window.h"
 
 namespace chrome {
@@ -32,7 +33,7 @@ class UserSwitchAnimatorChromeOS {
   };
 
   UserSwitchAnimatorChromeOS(MultiUserWindowManagerChromeOS* owner,
-                             const std::string& new_user_id,
+                             const user_manager::UserID& new_user_id,
                              int animation_speed_ms);
   ~UserSwitchAnimatorChromeOS();
 
@@ -45,7 +46,7 @@ class UserSwitchAnimatorChromeOS {
 
   // Returns the user id for which the wallpaper is currently shown.
   // If a wallpaper is transitioning to B it will be returned as "->B".
-  const std::string& wallpaper_user_id_for_test() { return wallpaper_user_id_; }
+  const std::string& wallpaper_user_email_for_test() { return wallpaper_user_email_; }
 
   // Advances the user switch animation to the next step. It reads the current
   // step from |animation_step_| and increments it thereafter. When
@@ -95,7 +96,7 @@ class UserSwitchAnimatorChromeOS {
   MultiUserWindowManagerChromeOS* owner_;
 
   // The new user to set.
-  std::string new_user_id_;
+  user_manager::UserID new_user_id_;
 
   // The animation speed in ms. If 0, animations are disabled.
   int animation_speed_ms_;
@@ -107,7 +108,7 @@ class UserSwitchAnimatorChromeOS {
   TransitioningScreenCover screen_cover_;
 
   // Mapping users IDs to the list of windows to show for these users.
-  typedef std::map<std::string, aura::Window::Windows> UserToWindowsMap;
+  typedef std::map<user_manager::UserID, aura::Window::Windows> UserToWindowsMap;
   UserToWindowsMap windows_by_user_id_;
 
   // A timer which watches to executes the second part of a "user changed"
@@ -115,7 +116,7 @@ class UserSwitchAnimatorChromeOS {
   scoped_ptr<base::Timer> user_changed_animation_timer_;
 
   // For unit tests: Check which wallpaper was set.
-  std::string wallpaper_user_id_;
+  std::string wallpaper_user_email_;
 
   DISALLOW_COPY_AND_ASSIGN(UserSwitchAnimatorChromeOS);
 };