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

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

Index: chrome/browser/ui/ash/multi_user/multi_user_window_manager_stub.h
diff --git a/chrome/browser/ui/ash/multi_user/multi_user_window_manager_stub.h b/chrome/browser/ui/ash/multi_user/multi_user_window_manager_stub.h
index 46df924d30e26f9ddb958e2d882968ece7679652..fcfef8cd5bea6c34b0a1804239f8c9a39f0a8817 100644
--- a/chrome/browser/ui/ash/multi_user/multi_user_window_manager_stub.h
+++ b/chrome/browser/ui/ash/multi_user/multi_user_window_manager_stub.h
@@ -12,6 +12,10 @@
 #include "base/compiler_specific.h"
 #include "chrome/browser/ui/ash/multi_user/multi_user_window_manager.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chrome {
 
 // This is the implementation of MultiUserWindowManager for single user mode.
@@ -22,16 +26,16 @@ class MultiUserWindowManagerStub : public MultiUserWindowManager {
 
   // MultiUserWindowManager overrides:
   void SetWindowOwner(aura::Window* window,
-                      const std::string& user_id) override;
-  const std::string& GetWindowOwner(aura::Window* window) const override;
+                      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;
+                         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;