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

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

Index: chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.h
diff --git a/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.h b/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.h
index 6ca57b6b7adbbb53e011de93d939eec00322073f..972c1f20d1e580e9e67a524bf5dfd0debe02c81e 100644
--- a/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.h
+++ b/chrome/browser/ui/ash/multi_user/multi_user_window_manager_chromeos.h
@@ -13,6 +13,7 @@
 #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"
@@ -69,7 +70,7 @@ class MultiUserWindowManagerChromeOS
   };
 
   // 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.
@@ -77,25 +78,25 @@ class MultiUserWindowManagerChromeOS
 
   // 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;
@@ -120,31 +121,31 @@ class MultiUserWindowManagerChromeOS
   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;
     }
 
@@ -153,10 +154,10 @@ class MultiUserWindowManagerChromeOS
 
    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_;
@@ -169,7 +170,7 @@ class MultiUserWindowManagerChromeOS
   // 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
@@ -191,7 +192,7 @@ class MultiUserWindowManagerChromeOS
   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.
@@ -242,7 +243,9 @@ class MultiUserWindowManagerChromeOS
   // 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.