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

components/proximity_auth/screenlock_bridge.h

 // Copyright 2014 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 COMPONENTS_PROXIMITY_AUTH_SCREENLOCK_BRIDGE_H_
 #define COMPONENTS_PROXIMITY_AUTH_SCREENLOCK_BRIDGE_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list.h"
 #include "base/strings/string16.h"
 #include "base/values.h"
 
+#include "components/user_manager/user_id.h"
+
 namespace content {
 class BrowserContext;
 }  // namespace content
 
 namespace proximity_auth {
 
 class ProximityAuthClient;
 
 // ScreenlockBridge brings together the screenLockPrivate API and underlying
 // support. On ChromeOS, it delegates calls to the ScreenLocker. On other
@@ skipping 80 matching lines @@
       FORCE_OFFLINE_PASSWORD = 5
     };
 
     enum ScreenType { SIGNIN_SCREEN = 0, LOCK_SCREEN = 1, OTHER_SCREEN = 2 };
 
     // Displays |message| in a banner on the lock screen.
     virtual void ShowBannerMessage(const base::string16& message) = 0;
 
     // Shows a custom icon in the user pod on the lock screen.
     virtual void ShowUserPodCustomIcon(
-        const std::string& user_email,
+        const user_manager::UserID& user_id,
         const UserPodCustomIconOptions& icon) = 0;
 
     // Hides the custom icon in user pod for a user.
-    virtual void HideUserPodCustomIcon(const std::string& user_email) = 0;
+    virtual void HideUserPodCustomIcon(const user_manager::UserID& user_id) = 0;
 
     // (Re)enable lock screen UI.
     virtual void EnableInput() = 0;
 
     // Set the authentication type to be used on the lock screen.
-    virtual void SetAuthType(const std::string& user_email,
+    virtual void SetAuthType(const user_manager::UserID& user_id,
                              AuthType auth_type,
                              const base::string16& auth_value) = 0;
 
     // Returns the authentication type used for a user.
-    virtual AuthType GetAuthType(const std::string& user_email) const = 0;
+    virtual AuthType GetAuthType(const user_manager::UserID& user_id) const = 0;
 
     // Returns the type of the screen -- a signin or a lock screen.
     virtual ScreenType GetScreenType() const = 0;
 
     // Unlocks from easy unlock app for a user.
-    virtual void Unlock(const std::string& user_email) = 0;
+    virtual void Unlock(const user_manager::UserID& user_id) = 0;
 
     // Attempts to login the user using an easy unlock key.
-    virtual void AttemptEasySignin(const std::string& user_email,
+    virtual void AttemptEasySignin(const user_manager::UserID& user_id,
                                    const std::string& secret,
                                    const std::string& key_label) = 0;
 
    protected:
     virtual ~LockHandler() {}
   };
 
   class Observer {
    public:
     // Invoked after the screen is locked.
     virtual void OnScreenDidLock(LockHandler::ScreenType screen_type) = 0;
 
     // Invoked after the screen lock is dismissed.
     virtual void OnScreenDidUnlock(LockHandler::ScreenType screen_type) = 0;
 
     // Invoked when the user focused on the lock screen changes.
-    virtual void OnFocusedUserChanged(const std::string& user_id) = 0;
+    virtual void OnFocusedUserChanged(const user_manager::UserID& user_id) = 0;
 
    protected:
     virtual ~Observer() {}
   };
 
   void SetLockHandler(LockHandler* lock_handler);
-  void SetFocusedUser(const std::string& user_id);
+  void SetFocusedUser(const user_manager::UserID& user_id);
 
   bool IsLocked() const;
   void Lock(content::BrowserContext* browser_context);
   void Unlock(content::BrowserContext* browser_context);
 
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
   LockHandler* lock_handler() { return lock_handler_; }
 
-  std::string focused_user_id() const { return focused_user_id_; }
+  const user_manager::UserID& focused_user_id() const { return focused_user_id_; }
 
  private:
   ProximityAuthClient* client_;  // Not owned. Must outlive this object.
   LockHandler* lock_handler_;    // Not owned
   // The last focused user's id.
-  std::string focused_user_id_;
+  user_manager::UserID focused_user_id_;
   ObserverList<Observer, true> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(ScreenlockBridge);
 };
 
 }  // namespace proximity_auth
 
 #endif  // COMPONENTS_PROXIMITY_AUTH_SCREENLOCK_BRIDGE_H_