Remove calls to deprecated MessageLoop methods in extensions.

extensions/browser/file_reader.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 "extensions/browser/file_reader.h"
 
 #include "base/bind.h"
 #include "base/files/file_util.h"
 #include "base/message_loop/message_loop.h"
+#include "base/single_thread_task_runner.h"
 #include "content/public/browser/browser_thread.h"
 
 using content::BrowserThread;
 
 FileReader::FileReader(const extensions::ExtensionResource& resource,
                        const Callback& callback)
     : resource_(resource),
       callback_(callback),
       origin_loop_(base::MessageLoop::current()) {}
 
 void FileReader::Start() {
   BrowserThread::PostTask(
       BrowserThread::FILE, FROM_HERE,
       base::Bind(&FileReader::ReadFileOnBackgroundThread, this));
 }
 
 FileReader::~FileReader() {}
 
 void FileReader::ReadFileOnBackgroundThread() {
   std::string data;
   bool success = base::ReadFileToString(resource_.GetFilePath(), &data);
-  origin_loop_->PostTask(FROM_HERE, base::Bind(callback_, success, data));
+  origin_loop_->task_runner()->PostTask(FROM_HERE,
+                                        base::Bind(callback_, success, data));
 }