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

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

 // Copyright 2013 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_MULTI_USER_WINDOW_MANAGER_CHROMEOS_H_
 #define CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_WINDOW_MANAGER_CHROMEOS_H_
 
 #include <map>
 #include <string>
 
 #include "ash/session/session_state_observer.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list.h"
 #include "chrome/browser/ui/ash/multi_user/multi_user_window_manager.h"
+#include "components/user_manager/user_id.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "ui/aura/window_observer.h"
 #include "ui/wm/core/transient_window_observer.h"
 
 class Browser;
 class MultiUserNotificationBlockerChromeOS;
 class MultiUserNotificationBlockerChromeOSTest;
 
 namespace content {
@@ skipping 36 matching lines @@
       public wm::TransientWindowObserver {
  public:
   // The speed which should be used to perform animations.
   enum AnimationSpeed {
     ANIMATION_SPEED_NORMAL,   // The normal animation speed.
     ANIMATION_SPEED_FAST,     // Unit test speed which test animations.
     ANIMATION_SPEED_DISABLED  // Unit tests which do not require animations.
   };
 
   // Create the manager and use |active_user_id| as the active user.
-  explicit MultiUserWindowManagerChromeOS(const std::string& active_user_id);
+  explicit MultiUserWindowManagerChromeOS(const user_manager::UserID& active_user_id);
   ~MultiUserWindowManagerChromeOS() override;
 
   // Initializes the manager after its creation. Should only be called once.
   void Init();
 
   // MultiUserWindowManager overrides:
   void SetWindowOwner(
-      aura::Window* window, const std::string& user_id) override;
-  const std::string& GetWindowOwner(
+      aura::Window* window, const user_manager::UserID& user_id) override;
+  const user_manager::UserID& GetWindowOwner(
       aura::Window* window) const override;
   void ShowWindowForUser(
-      aura::Window* window, const std::string& user_id) override;
+      aura::Window* window, const user_manager::UserID& user_id) override;
   bool AreWindowsSharedAmongUsers() const override;
   void GetOwnersOfVisibleWindows(
-      std::set<std::string>* user_ids) const override;
+      std::set<user_manager::UserID>* user_ids) const override;
   bool IsWindowOnDesktopOfUser(
       aura::Window* window,
-      const std::string& user_id) const override;
-  const std::string& GetUserPresentingWindow(
+      const user_manager::UserID& user_id) const override;
+  const user_manager::UserID& GetUserPresentingWindow(
       aura::Window* window) const override;
   void AddUser(content::BrowserContext* context) override;
   void AddObserver(Observer* observer) override;
   void RemoveObserver(Observer* observer) override;
 
   // SessionStateObserver overrides:
-  void ActiveUserChanged(const std::string& user_id) override;
+  void ActiveUserChanged(const user_manager::UserID& user_id) override;
 
   // WindowObserver overrides:
   void OnWindowDestroyed(aura::Window* window) override;
   void OnWindowVisibilityChanging(aura::Window* window, bool visible) override;
   void OnWindowVisibilityChanged(aura::Window* window, bool visible) override;
 
   // TransientWindowObserver overrides:
   void OnTransientChildAdded(aura::Window* window,
                              aura::Window* transient) override;
   void OnTransientChildRemoved(aura::Window* window,
                                aura::Window* transient) override;
 
   // content::NotificationObserver overrides:
   void Observe(int type,
                const content::NotificationSource& source,
                const content::NotificationDetails& details) override;
 
   // Disable any animations for unit tests.
   void SetAnimationSpeedForTest(AnimationSpeed speed);
 
   // Returns true when a user switch animation is running. For unit tests.
   bool IsAnimationRunningForTest();
 
   // Returns the current user for unit tests.
-  const std::string& GetCurrentUserForTest() const;
+  const user_manager::UserID& GetCurrentUserForTest() const;
 
  protected:
   friend class UserSwitchAnimatorChromeOS;
 
   class WindowEntry {
    public:
-    explicit WindowEntry(const std::string& user_id)
+    explicit WindowEntry(const user_manager::UserID& user_id)
         : owner_(user_id),
           show_for_user_(user_id),
           show_(true) {}
     virtual ~WindowEntry() {}
 
     // Returns the owner of this window. This cannot be changed.
-    const std::string& owner() const { return owner_; }
+    const user_manager::UserID& owner() const { return owner_; }
 
     // Returns the user for which this should be shown.
-    const std::string& show_for_user() const { return show_for_user_; }
+    const user_manager::UserID& show_for_user() const { return show_for_user_; }
 
     // Returns if the window should be shown for the "show user" or not.
     bool show() const { return show_; }
 
     // Set the user which will display the window on the owned desktop. If
     // an empty user id gets passed the owner will be used.
-    void set_show_for_user(const std::string& user_id) {
+    void set_show_for_user(const user_manager::UserID& user_id) {
       show_for_user_ = user_id.empty() ? owner_ : user_id;
     }
 
     // Sets if the window gets shown for the active user or not.
     void set_show(bool show) { show_ = show; }
 
    private:
     // The user id of the owner of this window.
-    const std::string owner_;
+    const user_manager::UserID owner_;
 
     // The user id of the user on which desktop the window gets shown.
-    std::string show_for_user_;
+    user_manager::UserID show_for_user_;
 
     // True if the window should be visible for the user which shows the window.
     bool show_;
 
     DISALLOW_COPY_AND_ASSIGN(WindowEntry);
   };
 
   typedef std::map<aura::Window*, WindowEntry*> WindowToEntryMap;
 
   // Show a window for a user without switching the user.
   // Returns true when the window moved to a new desktop.
   bool ShowWindowForUserIntern(aura::Window* window,
-                               const std::string& user_id);
+                               const user_manager::UserID& user_id);
 
   // Show / hide the given window. Note: By not doing this within the functions,
   // this allows to either switching to different ways to show/hide and / or to
   // distinguish state changes performed by this class vs. state changes
   // performed by the others. Note furthermore that system modal dialogs will
   // not get hidden. We will switch instead to the owners desktop.
   // The |animation_time_in_ms| is the time the animation should take. Set to 0
   // if it should get set instantly.
   void SetWindowVisibility(aura::Window* window,
                            bool visible,
                            int animation_time_in_ms);
 
   const WindowToEntryMap& window_to_entry() { return window_to_entry_; }
   MultiUserNotificationBlockerChromeOS* notification_blocker() {
     return notification_blocker_.get();
   }
 
  private:
   friend class ::MultiUserNotificationBlockerChromeOSTest;
   friend class ash::test::MultiUserWindowManagerChromeOSTest;
 
-  typedef std::map<std::string, AppObserver*> UserIDToAppWindowObserver;
+  typedef std::map<user_manager::UserID, AppObserver*> UserIDToAppWindowObserver;
   typedef std::map<aura::Window*, bool> TransientWindowToVisibility;
 
   // Add a browser window to the system so that the owner can be remembered.
   void AddBrowserWindow(Browser* browser);
 
   // Show the window and its transient children. However - if a transient child
   // was turned invisible by some other operation, it will stay invisible.
   // Use the given |animation_time_in_ms| for transitioning.
   void ShowWithTransientChildrenRecursive(aura::Window* window,
                                           int animation_time_in_ms);
@@ skipping 30 matching lines @@
 
   // An observer list to be notified upon window owner changes.
   ObserverList<Observer> observers_;
 
   // A map which remembers for owned transient windows their own visibility.
   TransientWindowToVisibility transient_window_to_visibility_;
 
   // The currently selected active user. It is used to find the proper
   // visibility state in various cases. The state is stored here instead of
   // being read from the user manager to be in sync while a switch occurs.
-  std::string current_user_id_;
+  user_manager::UserID current_user_id_;
+
+  const user_manager::UserID null_user_id_;
 
   // The blocker which controls the desktop notification visibility based on the
   // current multi-user status.
   scoped_ptr<MultiUserNotificationBlockerChromeOS> notification_blocker_;
 
   // The notification registrar to track the creation of browser windows.
   content::NotificationRegistrar registrar_;
 
   // Suppress changes to the visibility flag while we are changing it ourselves.
   bool suppress_visibility_changes_;
 
   // The speed which is used to perform any animations.
   AnimationSpeed animation_speed_;
 
   // The animation between users.
   scoped_ptr<UserSwitchAnimatorChromeOS> animation_;
 
   DISALLOW_COPY_AND_ASSIGN(MultiUserWindowManagerChromeOS);
 };
 
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_WINDOW_MANAGER_CHROMEOS_H_