Make AudioDebugFileWriter create its own file worker thread.

media/audio/audio_manager_base.h

Index: media/audio/audio_manager_base.h
diff --git a/media/audio/audio_manager_base.h b/media/audio/audio_manager_base.h
index ebb37792dcd66d1e1bb680e63ce759c8aae54c66..cc1d10aba0bcf1c240679b73a228ea3ae8bb563b 100644
--- a/media/audio/audio_manager_base.h
+++ b/media/audio/audio_manager_base.h
@@ -152,8 +152,7 @@ class MEDIA_EXPORT AudioManagerBase : public AudioManager {
 
   virtual std::unique_ptr<AudioDebugRecordingManager>
   CreateAudioDebugRecordingManager(
-      scoped_refptr<base::SingleThreadTaskRunner> task_runner,
-      scoped_refptr<base::SingleThreadTaskRunner> file_task_runner);
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner);
 
  private:
   FRIEND_TEST_ALL_PREFIXES(AudioManagerTest, AudioDebugRecording);
@@ -164,8 +163,7 @@ class MEDIA_EXPORT AudioManagerBase : public AudioManager {
   class CompareByParams;
 
   // AudioManager:
-  void InitializeOutputDebugRecording(
-      scoped_refptr<base::SingleThreadTaskRunner> file_task_runner) final;
+  void InitializeOutputDebugRecording() final;
 
   // These functions assign group ids to devices based on their device ids.
   // The default implementation is an attempt to do this based on