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

chrome/browser/ui/webui/options/chromeos/user_image_source.h

 // Copyright (c) 2012 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_UI_WEBUI_OPTIONS_CHROMEOS_USER_IMAGE_SOURCE_H_
 #define CHROME_BROWSER_UI_WEBUI_OPTIONS_CHROMEOS_USER_IMAGE_SOURCE_H_
 
 #include <string>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "content/public/browser/url_data_source.h"
 #include "ui/base/layout.h"
 
 namespace base {
 class RefCountedMemory;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 namespace options {
 
 // UserImageSource is the data source that serves user images for users that
 // have it.
 class UserImageSource : public content::URLDataSource {
  public:
   UserImageSource();
 
   // content::URLDataSource implementation.
   std::string GetSource() const override;
   void StartDataRequest(
       const std::string& path,
       int render_process_id,
       int render_frame_id,
       const content::URLDataSource::GotDataCallback& callback) override;
   std::string GetMimeType(const std::string& path) const override;
 
   // Returns PNG encoded image for user with specified email. If there's
-  // no user with such email, returns the first default image.
-  static base::RefCountedMemory* GetUserImage(const std::string& email,
+  // no user with such user id, returns the first default image.
+  static base::RefCountedMemory* GetUserImage(const user_manager::UserID& user_id,
                                               ui::ScaleFactor scale_factor);
 
  private:
   ~UserImageSource() override;
 
   DISALLOW_COPY_AND_ASSIGN(UserImageSource);
 };
 
 }  // namespace options
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_UI_WEBUI_OPTIONS_CHROMEOS_USER_IMAGE_SOURCE_H_