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

chrome/browser/ui/ash/multi_user/multi_user_window_manager.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_H_
 #define CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_WINDOW_MANAGER_H_
 
 #include <map>
 #include <set>
 #include <string>
 
 class Browser;
 
 namespace content {
 class BrowserContext;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace aura {
 class Window;
 }
 
 namespace chrome {
 
 class MultiUserWindowManagerChromeOS;
 
 // The MultiUserWindowManager manages windows from multiple users by presenting
 // only user relevant windows to the current user. The manager is automatically
@@ skipping 62 matching lines @@
   // with a given |mode| for test purposes.
   static void SetInstanceForTest(MultiUserWindowManager* instance,
                                  MultiProfileMode mode);
 
   // Assigns an owner to a passed window. Note that this window's parent should
   // be a direct child of the root window.
   // A user switch will automatically change the visibility - and - if the
   // current user is not the owner it will immediately hidden. If the window
   // had already be registered this function will run into a DCHECK violation.
   virtual void SetWindowOwner(
-      aura::Window* window, const std::string& user_id) = 0;
+      aura::Window* window, const user_manager::UserID& user_id) = 0;
 
   // See who owns this window. The return value is the user id or an empty
   // string if not assigned yet.
-  virtual const std::string& GetWindowOwner(aura::Window* window) const = 0;
+  virtual const user_manager::UserID& GetWindowOwner(aura::Window* window) const = 0;
 
   // Allows to show an owned window for another users. If the window is not
   // owned, this call will return immediately. (The FileManager for example
   // might be available for every user and not belong explicitly to one).
   // Note that a window can only be shown on one desktop at a time. Note that
   // when the window gets minimized, it will automatically fall back to the
   // owner's desktop.
   virtual void ShowWindowForUser(
-      aura::Window* window, const std::string& user_id) = 0;
+      aura::Window* window, const user_manager::UserID& user_id) = 0;
 
   // Returns true when windows are shared among users.
   virtual bool AreWindowsSharedAmongUsers() const = 0;
 
   // Get the owners for the visible windows and set them to |user_ids|.
   virtual void GetOwnersOfVisibleWindows(
-      std::set<std::string>* user_ids) const = 0;
+      std::set<user_manager::UserID>* user_ids) const = 0;
 
   // A query call for a given window to see if it is on the given user's
   // desktop.
   virtual bool IsWindowOnDesktopOfUser(aura::Window* window,
-                                       const std::string& user_id) const = 0;
+                                       const user_manager::UserID& user_id) const = 0;
 
   // Get the user on which the window is currently shown. If an empty string is
   // passed back the window will be presented for every user.
-  virtual const std::string& GetUserPresentingWindow(
+  virtual const user_manager::UserID& GetUserPresentingWindow(
       aura::Window* window) const = 0;
 
   // Adds user to monitor starting and running V1/V2 application windows.
   // Returns immediately if the user (identified by a |profile|) is already
   // known to the manager. Note: This function is not implemented as a
   // SessionStateObserver to coordinate the timing of the addition with other
   // modules.
   virtual void AddUser(content::BrowserContext* profile) = 0;
 
   // Manages observers.
   virtual void AddObserver(Observer* observer) = 0;
   virtual void RemoveObserver(Observer* observer) = 0;
 
  protected:
   virtual ~MultiUserWindowManager() {}
 
  private:
   // Caching the current multi profile mode since the detection is expensive.
   static MultiProfileMode multi_user_mode_;
 };
 
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_WINDOW_MANAGER_H_