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

chrome/browser/chromeos/login/users/avatar/user_image_manager_impl.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_IMPL_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_USERS_AVATAR_USER_IMAGE_MANAGER_IMPL_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 13 matching lines @@
 #include "ui/gfx/image/image_skia.h"
 
 class ProfileDownloader;
 
 namespace base {
 class FilePath;
 class SequencedTaskRunner;
 }
 
 namespace user_manager {
+class UserID;
 class UserImage;
 class UserManager;
 }
 
 namespace chromeos {
 
 class UserImageSyncObserver;
 
 class UserImageManagerImpl
     : public UserImageManager,
       public ProfileDownloaderDelegate {
  public:
   // UserImageManager:
-  UserImageManagerImpl(const std::string& user_id,
+  UserImageManagerImpl(const user_manager::UserID& user_id,
                        user_manager::UserManager* user_manager);
   ~UserImageManagerImpl() override;
 
   void LoadUserImage() override;
   void UserLoggedIn(bool user_is_new, bool user_is_local) override;
   void UserProfileCreated() override;
   void SaveUserDefaultImageIndex(int default_image_index) override;
   void SaveUserImage(const user_manager::UserImage& user_image) override;
   void SaveUserImageFromFile(const base::FilePath& path) override;
   void SaveUserImageFromProfileImage() override;
@@ skipping 160 matching lines @@
   bool user_needs_migration_;
 
   base::WeakPtrFactory<UserImageManagerImpl> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(UserImageManagerImpl);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_USERS_AVATAR_USER_IMAGE_MANAGER_IMPL_H_