Make MediaFileUtil run on thread pool.

webkit/fileapi/file_system_operation_context.cc

 // 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.
 
 #include "webkit/fileapi/file_system_operation_context.h"
 
+#include "base/sequenced_task_runner.h"
 #include "webkit/fileapi/file_system_context.h"
 
 namespace fileapi {
 
 FileSystemOperationContext::FileSystemOperationContext(
     FileSystemContext* context)
     : file_system_context_(context),
+      task_runner_(file_system_context_->file_task_runner()),
       allowed_bytes_growth_(0) {}
 
 FileSystemOperationContext::~FileSystemOperationContext() {}
 
-base::SequencedTaskRunner*
-FileSystemOperationContext::file_task_runner() const {
-  return file_system_context_->file_task_runner();
+void FileSystemOperationContext::SetTaskRunner(
+    base::SequencedTaskRunner* task_runner) {
+  task_runner_ = task_runner;
 }
 
 }  // namespace fileapi