Make AudioDebugFileWriter create its own file worker thread.

content/browser/renderer_host/media/audio_input_renderer_host.cc

Index: content/browser/renderer_host/media/audio_input_renderer_host.cc
diff --git a/content/browser/renderer_host/media/audio_input_renderer_host.cc b/content/browser/renderer_host/media/audio_input_renderer_host.cc
index 64c1a6169988c3430221a478f5f7c953b84e8e5a..9b3459e430de2cf31eabee53b76086acddd250a9 100644
--- a/content/browser/renderer_host/media/audio_input_renderer_host.cc
+++ b/content/browser/renderer_host/media/audio_input_renderer_host.cc
@@ -355,7 +355,6 @@ void AudioInputRendererHost::DoCreateStream(
             device_id, audio_params, audio_manager_->GetWorkerTaskRunner(),
             audio_mirroring_manager_),
         entry->writer.get(), user_input_monitor_,
-        BrowserThread::GetTaskRunnerForThread(BrowserThread::FILE),
         audio_params);
     // Only count for captures from desktop media picker dialog.
     if (entry->controller.get() && type == MEDIA_DESKTOP_AUDIO_CAPTURE)
@@ -363,8 +362,7 @@ void AudioInputRendererHost::DoCreateStream(
   } else {
     entry->controller = media::AudioInputController::Create(
         audio_manager_, this, entry->writer.get(), user_input_monitor_,
-        audio_params, device_id, config.automatic_gain_control,
-        BrowserThread::GetTaskRunnerForThread(BrowserThread::FILE));
+        audio_params, device_id, config.automatic_gain_control);
     oss << ", AGC=" << config.automatic_gain_control;
 
     // Only count for captures from desktop media picker dialog and system loop