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

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

 // 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.
 
 #ifndef CHROME_BROWSER_UI_ASH_MULTI_USER_USER_SWITCH_ANIMATOR_CHROMEOS_H_
 #define CHROME_BROWSER_UI_ASH_MULTI_USER_USER_SWITCH_ANIMATOR_CHROMEOS_H_
 
 #include <map>
 #include <string>
 
 #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 {
 
 class MultiUserWindowManagerChromeOS;
 
 // A class which performs transitions animations between users. Upon creation,
 // the animation gets started and upon destruction the animation gets finished
 // if not done yet.
 // Specifying |animation_disabled| upon creation will perform the transition
 // without visible animations.
 class UserSwitchAnimatorChromeOS {
  public:
   // The animation step for the user change animation.
   enum AnimationStep {
     ANIMATION_STEP_HIDE_OLD_USER,  // Hiding the old user (and shelf).
     ANIMATION_STEP_SHOW_NEW_USER,  // Show the shelf of the new user.
     ANIMATION_STEP_FINALIZE,       // All animations are done - final cleanup.
     ANIMATION_STEP_ENDED           // The animation has ended.
   };
 
   UserSwitchAnimatorChromeOS(MultiUserWindowManagerChromeOS* owner,
-                             const std::string& new_user_id,
+                             const user_manager::UserID& new_user_id,
                              int animation_speed_ms);
   ~UserSwitchAnimatorChromeOS();
 
   // Check if a window is covering the entire work area of the screen it is on.
   static bool CoversScreen(aura::Window* window);
 
   bool IsAnimationFinished() {
     return animation_step_ == ANIMATION_STEP_ENDED;
   }
 
   // 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
   // |ANIMATION_STEP_FINALIZE| gets executed, the animation is finished and the
   // timer (if one exists) will get destroyed.
   void AdvanceUserTransitionAnimation();
 
   // When the system is shutting down, the animation can be stopped without
   // ending it.
   void CancelAnimation();
@@ skipping 29 matching lines @@
 
   // Builds the map that a user ID to the list of windows that should be shown
   // for this user. This operation happens once upon the construction of this
   // animation.
   void BuildUserToWindowsListMap();
 
   // The owning window manager.
   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_;
 
   // The next animation step for AdvanceUserTransitionAnimation().
   AnimationStep animation_step_;
 
   // The screen cover status before the animation has started.
   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"
   // animation. Note that this timer exists only during such an animation.
   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);
 };
 
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_UI_ASH_MULTI_USER_USER_SWITCH_ANIMATOR_CHROMEOS_H_