Make AudioDebugFileWriter create its own file worker thread.

content/browser/browser_main_loop.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 "content/browser/browser_main_loop.h"
 
 #include <stddef.h>
 
 #include <string>
 #include <utility>
@@ skipping 1758 matching lines @@
 }
 
 void BrowserMainLoop::CreateAudioManager() {
   DCHECK(!audio_manager_);
 
   audio_manager_ = GetContentClient()->browser()->CreateAudioManager(
       MediaInternals::GetInstance());
   if (!audio_manager_) {
     audio_manager_ = media::AudioManager::Create(
         base::MakeUnique<media::AudioThreadImpl>(),
-        BrowserThread::GetTaskRunnerForThread(BrowserThread::FILE),
         MediaInternals::GetInstance());
   }
   CHECK(audio_manager_);
 
   audio_system_ = media::AudioSystemImpl::Create(audio_manager_.get());
   CHECK(audio_system_);
 }
 
 }  // namespace content