Fix missing #include build error from r251504.

chrome/browser/media_galleries/media_folder_finder.cc

 // 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.
 
 #include "chrome/browser/media_galleries/media_folder_finder.h"
 
 #include <algorithm>
 #include <set>
 
 #include "base/files/file_enumerator.h"
 #include "base/path_service.h"
 #include "base/sequence_checker.h"
 #include "base/stl_util.h"
 #include "base/strings/string_util.h"
 #include "base/task_runner_util.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "chrome/browser/media_galleries/fileapi/media_path_filter.h"
 #include "chrome/browser/storage_monitor/storage_monitor.h"
 #include "content/public/browser/browser_thread.h"
 
 #if defined(OS_CHROMEOS)
 #include "chrome/common/chrome_paths.h"
 #include "chromeos/dbus/cros_disks_client.h"
 #endif
 
 typedef base::Callback<void(const std::vector<base::FilePath>& /*roots*/)>
@@ skipping 302 matching lines @@
 
   if (!IsEmptyScanResult(reply.scan_result))
     results_[path] = reply.scan_result;
 
   // Push new folders to the |folders_to_scan_| in reverse order.
   std::copy(reply.new_folders.rbegin(), reply.new_folders.rend(),
             std::back_inserter(folders_to_scan_));
 
   ScanFolder();
 }