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

chrome/browser/chromeos/login/users/avatar/user_image_manager.h

Index: chrome/browser/chromeos/login/users/avatar/user_image_manager.h
diff --git a/chrome/browser/chromeos/login/users/avatar/user_image_manager.h b/chrome/browser/chromeos/login/users/avatar/user_image_manager.h
index f8f121fe337ee5d5b231ce916bda73ae691cd259..1250c8b99d51bd869c847ecb3dcf83ec259062d5 100644
--- a/chrome/browser/chromeos/login/users/avatar/user_image_manager.h
+++ b/chrome/browser/chromeos/login/users/avatar/user_image_manager.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "components/user_manager/user.h"
+#include "components/user_manager/user_id.h"
 
 class PrefRegistrySimple;
 
@@ -34,7 +35,7 @@ class UserImageManager {
   // 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.
@@ -101,11 +102,11 @@ class UserImageManager {
                                      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