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

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

Index: chrome/browser/ui/ash/multi_user/multi_user_window_manager.h
diff --git a/chrome/browser/ui/ash/multi_user/multi_user_window_manager.h b/chrome/browser/ui/ash/multi_user/multi_user_window_manager.h
index 2496ea52d60dd270879365505d0af3505064e5ac..4cfd1bbf5770e6f9f396e2a0c0e3c43ab8085543 100644
--- a/chrome/browser/ui/ash/multi_user/multi_user_window_manager.h
+++ b/chrome/browser/ui/ash/multi_user/multi_user_window_manager.h
@@ -15,6 +15,10 @@ namespace content {
 class BrowserContext;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace aura {
 class Window;
 }
@@ -97,11 +101,11 @@ class MultiUserWindowManager {
   // 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
@@ -110,23 +114,23 @@ class MultiUserWindowManager {
   // 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.