Use worker pool for IO in JsonPrefStore.

chrome/service/service_process_prefs.cc

 // 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.
 
 #include "chrome/service/service_process_prefs.h"
 
 #include "base/values.h"
 
 ServiceProcessPrefs::ServiceProcessPrefs(
     const FilePath& pref_filename,
-    base::MessageLoopProxy* file_message_loop_proxy)
-    : prefs_(new JsonPrefStore(pref_filename, file_message_loop_proxy)) {
+    base::SequencedTaskRunner* blocking_task_runner)
+    : prefs_(new JsonPrefStore(pref_filename, blocking_task_runner)) {
 }
 
 ServiceProcessPrefs::~ServiceProcessPrefs() {}
 
 void ServiceProcessPrefs::ReadPrefs() {
   prefs_->ReadPrefs();
 }
 
 void ServiceProcessPrefs::WritePrefs() {
   prefs_->CommitPendingWrite();
@@ skipping 28 matching lines @@
       !value->IsType(Value::TYPE_DICTIONARY)) {
     return;
   }
 
   *result = static_cast<const DictionaryValue*>(value);
 }
 
 void ServiceProcessPrefs::RemovePref(const std::string& key) {
   prefs_->RemoveValue(key);
 }