Use worker pool for IO in JsonPrefStore.

chrome/common/json_pref_store.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_COMMON_JSON_PREF_STORE_H_
 #define CHROME_COMMON_JSON_PREF_STORE_H_
 #pragma once
 
 #include <set>
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/file_path.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/observer_list.h"
 #include "chrome/common/important_file_writer.h"
 #include "chrome/common/persistent_pref_store.h"
 
 namespace base {
 class DictionaryValue;
-class MessageLoopProxy;
+class SequencedTaskRunner;
 class Value;
 }
 
 class FilePath;
 
 // A writable PrefStore implementation that is used for user preferences.
 class JsonPrefStore : public PersistentPrefStore,
                       public ImportantFileWriter::DataSerializer {
  public:
-  // |file_message_loop_proxy| is the MessageLoopProxy for a thread on which
-  // file I/O can be done.
+  // |blocking_task_runner| is the SequencedTaskRunner on which file
+  // I/O can be done.
   JsonPrefStore(const FilePath& pref_filename,
-                base::MessageLoopProxy* file_message_loop_proxy);
+                base::SequencedTaskRunner* blocking_task_runner);
 
   // PrefStore overrides:
   virtual ReadResult GetValue(const std::string& key,
                               const base::Value** result) const OVERRIDE;
   virtual void AddObserver(PrefStore::Observer* observer) OVERRIDE;
   virtual void RemoveObserver(PrefStore::Observer* observer) OVERRIDE;
   virtual size_t NumberOfObservers() const OVERRIDE;
   virtual bool IsInitializationComplete() const OVERRIDE;
 
   // PersistentPrefStore overrides:
@@ skipping 17 matching lines @@
   // (read: do not call it manually).
   void OnFileRead(base::Value* value_owned, PrefReadError error, bool no_dir);
 
  private:
   virtual ~JsonPrefStore();
 
   // ImportantFileWriter::DataSerializer overrides:
   virtual bool SerializeData(std::string* output) OVERRIDE;
 
   FilePath path_;
-  scoped_refptr<base::MessageLoopProxy> file_message_loop_proxy_;
+  scoped_refptr<base::SequencedTaskRunner> blocking_task_runner_;
 
   scoped_ptr<base::DictionaryValue> prefs_;
 
   bool read_only_;
 
   // Helper for safely writing pref data.
   ImportantFileWriter writer_;
 
   ObserverList<PrefStore::Observer, true> observers_;
 
   scoped_ptr<ReadErrorDelegate> error_delegate_;
 
   bool initialized_;
   PrefReadError read_error_;
 
   std::set<std::string> keys_need_empty_value_;
 
   DISALLOW_COPY_AND_ASSIGN(JsonPrefStore);
 };
 
 #endif  // CHROME_COMMON_JSON_PREF_STORE_H_