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

chrome/browser/chromeos/extensions/wallpaper_api.cc

 // Copyright 2013 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.
 
 #include "chrome/browser/chromeos/extensions/wallpaper_api.h"
 
 #include <string>
 #include <vector>
 
 #include "ash/desktop_background/desktop_background_controller.h"
@@ skipping 75 matching lines @@
 
   scoped_ptr<net::URLFetcher> url_fetcher_;
   FetchCallback callback_;
 };
 
 base::LazyInstance<WallpaperFetcher> g_wallpaper_fetcher =
     LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace
 
-WallpaperSetWallpaperFunction::WallpaperSetWallpaperFunction() {
+WallpaperSetWallpaperFunction::WallpaperSetWallpaperFunction() : user_id_(std::string(), std::string()) {

[Denis Kuznetsov (DE-MUC), 2015/06/10 16:50:45]

EmptyUserID() ?

 }
 
 WallpaperSetWallpaperFunction::~WallpaperSetWallpaperFunction() {
 }
 
 bool WallpaperSetWallpaperFunction::RunAsync() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   params_ = set_wallpaper::Params::Create(*args_);
   EXTENSION_FUNCTION_VALIDATE(params_);
 
-  // Gets email address and username hash while at UI thread.
-  user_id_ = user_manager::UserManager::Get()->GetLoggedInUser()->email();
+  // Gets user id and username hash while at UI thread.
+  user_id_ = user_manager::UserManager::Get()->GetLoggedInUser()->GetUserID();
   user_id_hash_ =
       user_manager::UserManager::Get()->GetLoggedInUser()->username_hash();
 
   if (params_->details.data) {
     StartDecode(*params_->details.data);
   } else {
     GURL wallpaper_url(*params_->details.url);
     if (wallpaper_url.is_valid()) {
       g_wallpaper_fetcher.Get().FetchWallpaper(
           wallpaper_url,
@@ skipping 16 matching lines @@
 
   sequence_token_ = BrowserThread::GetBlockingPool()->GetNamedSequenceToken(
       wallpaper::kWallpaperSequenceTokenName);
   scoped_refptr<base::SequencedTaskRunner> task_runner =
       BrowserThread::GetBlockingPool()->
           GetSequencedTaskRunnerWithShutdownBehavior(sequence_token_,
               base::SequencedWorkerPool::BLOCK_SHUTDOWN);
   wallpaper::WallpaperLayout layout = wallpaper_api_util::GetLayoutEnum(
       extensions::api::wallpaper::ToString(params_->details.layout));
   bool update_wallpaper =
-      user_id_ == user_manager::UserManager::Get()->GetActiveUser()->email();
+      user_id_ == user_manager::UserManager::Get()->GetActiveUser()->GetUserID();
   wallpaper_manager->SetCustomWallpaper(user_id_,
                                         user_id_hash_,
                                         params_->details.filename,
                                         layout,
                                         user_manager::User::CUSTOMIZED,
                                         image,
                                         update_wallpaper);
   unsafe_wallpaper_decoder_ = NULL;
 
   if (params_->details.thumbnail) {
@@ skipping 67 matching lines @@
     const std::string& response) {
   if (success) {
     params_->details.data.reset(
         new std::vector<char>(response.begin(), response.end()));
     StartDecode(*params_->details.data);
   } else {
     SetError(response);
     SendResponse(false);
   }
 }