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

chrome/browser/chromeos/login/users/avatar/user_image_manager.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 CHROME_BROWSER_CHROMEOS_LOGIN_USERS_AVATAR_USER_IMAGE_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_USERS_AVATAR_USER_IMAGE_MANAGER_H_
 
 #include <string>
 
 #include "components/user_manager/user.h"
+#include "components/user_manager/user_id.h"
 
 class PrefRegistrySimple;
 
 namespace base {
 class FilePath;
 }
 
 namespace gfx {
 class ImageSkia;
 }
 
 namespace user_manager {
 class UserImage;
 }
 
 namespace chromeos {
 
 class UserImageSyncObserver;
 
 // Base class that provides a mechanism for updating user images.
 // There is an instance of this class for each user in the system.
 class UserImageManager {
  public:
   // Registers user image manager preferences.
   static void RegisterPrefs(PrefRegistrySimple* registry);
 
-  explicit UserImageManager(const std::string& user_id);
+  explicit UserImageManager(const user_manager::UserID& user_id);
   virtual ~UserImageManager();
 
   // Loads user image data from Local State.
   virtual void LoadUserImage() = 0;
 
   // Indicates that a user has just logged in.
   virtual void UserLoggedIn(bool user_is_new, bool user_is_local) = 0;
 
   // Indicates that a user profile was created.
   virtual void UserProfileCreated() = 0;
@@ skipping 46 matching lines @@
 
   // Invoked when the external data referenced for the user has been
   // fetched.  Failed fetches are retried and the method is called only
   // when a fetch eventually succeeds. If a fetch fails permanently
   // (e.g. because the external data reference specifies an invalid URL),
   // the method is not called at all.
   virtual void OnExternalDataFetched(const std::string& policy,
                                      scoped_ptr<std::string> data) = 0;
 
  protected:
-  const std::string& user_id() const { return user_id_; }
+  const user_manager::UserID& user_id() const { return user_id_; }
 
   // ID of user which images are managed by current instance of
   // UserImageManager.
-  const std::string user_id_;
+  const user_manager::UserID user_id_;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_USERS_AVATAR_USER_IMAGE_MANAGER_H_