Use worker pool for IO in JsonPrefStore.

chrome/common/important_file_writer.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_COMMON_IMPORTANT_FILE_WRITER_H_
 #define CHROME_COMMON_IMPORTANT_FILE_WRITER_H_
 #pragma once
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/file_path.h"
 #include "base/memory/ref_counted.h"
 #include "base/threading/non_thread_safe.h"
 #include "base/time.h"
 #include "base/timer.h"
 
 namespace base {
-class MessageLoopProxy;
+class SequencedTaskRunner;
 class Thread;
-}
+}  // namespace base
 
 // Helper to ensure that a file won't be corrupted by the write (for example on
 // application crash). Consider a naive way to save an important file F:
 //
 // 1. Open F for writing, truncating it.
 // 2. Write new data to F.
 //
 // It's good when it works, but it gets very bad if step 2. doesn't complete.
 // It can be caused by a crash, a computer hang, or a weird I/O error. And you
 // end up with a broken file.
@@ skipping 17 matching lines @@
     // ImportantFileWriter has been created.
     virtual bool SerializeData(std::string* data) = 0;
   };
 
   // Initialize the writer.
   // |path| is the name of file to write.
   // |file_message_loop_proxy| is the MessageLoopProxy for a thread on which
   // file I/O can be done.
   // All non-const methods, ctor and dtor must be called on the same thread.
   ImportantFileWriter(const FilePath& path,
-                      base::MessageLoopProxy* file_message_loop_proxy);
+                      base::SequencedTaskRunner* blocking_task_runner);
 
   // You have to ensure that there are no pending writes at the moment
   // of destruction.
   ~ImportantFileWriter();
 
   const FilePath& path() const { return path_; }
 
   // Returns true if there is a scheduled write pending which has not yet
   // been started.
   bool HasPendingWrite() const;
@@ skipping 18 matching lines @@
   }
 
   void set_commit_interval(const base::TimeDelta& interval) {
     commit_interval_ = interval;
   }
 
  private:
   // Path being written to.
   const FilePath path_;
 
-  // MessageLoopProxy for the thread on which file I/O can be done.
-  scoped_refptr<base::MessageLoopProxy> file_message_loop_proxy_;
+  // SequencedTaskRunner for blocking I/O operations.
+  scoped_refptr<base::SequencedTaskRunner> blocking_task_runner_;
 
   // Timer used to schedule commit after ScheduleWrite.
   base::OneShotTimer<ImportantFileWriter> timer_;
 
   // Serializer which will provide the data to be saved.
   DataSerializer* serializer_;
 
   // Time delta after which scheduled data will be written to disk.
   base::TimeDelta commit_interval_;
 
   DISALLOW_COPY_AND_ASSIGN(ImportantFileWriter);
 };
 
 #endif  // CHROME_COMMON_IMPORTANT_FILE_WRITER_H_