Use worker pool for IO in JsonPrefStore.

chrome/service/service_process_prefs.h

 // Copyright (c) 2011 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_SERVICE_SERVICE_PROCESS_PREFS_H_
 #define CHROME_SERVICE_SERVICE_PROCESS_PREFS_H_
 #pragma once
 
 #include <string>
 
 #include "chrome/common/json_pref_store.h"
 
 namespace base {
 class DictionaryValue;
-}
+}  // namespace base
 
 // Manages persistent preferences for the service process. This is basically a
 // thin wrapper around JsonPrefStore for more comfortable use.
 class ServiceProcessPrefs {
  public:
   // |file_message_loop_proxy| is the MessageLoopProxy for a thread on which
   // file I/O can be done.
   ServiceProcessPrefs(const FilePath& pref_filename,
-                      base::MessageLoopProxy* file_message_loop_proxy);
+                      base::SequencedTaskRunner* blocking_task_runner);
   ~ServiceProcessPrefs();
 
   // Read preferences from the backing file.
   void ReadPrefs();
 
   // Write the data to the backing file.
   void WritePrefs();
 
   // Get a string preference for |key| and store it in |result|.
   void GetString(const std::string& key, std::string* result);
@@ skipping 14 matching lines @@
   // Removes the pref specified by |key|.
   void RemovePref(const std::string& key);
 
  private:
   scoped_refptr<JsonPrefStore> prefs_;
 
   DISALLOW_COPY_AND_ASSIGN(ServiceProcessPrefs);
 };
 
 #endif  // CHROME_SERVICE_SERVICE_PROCESS_PREFS_H_