Make AudioDebugFileWriter create its own file worker thread.

media/audio/mock_audio_manager.cc

Index: media/audio/mock_audio_manager.cc
diff --git a/media/audio/mock_audio_manager.cc b/media/audio/mock_audio_manager.cc
index af1c643b2a0337aba3e5c6bde8350649e0f7a06b..d05921c846cbbb0b6a6fcf19931d3b0f4f462e66 100644
--- a/media/audio/mock_audio_manager.cc
+++ b/media/audio/mock_audio_manager.cc
@@ -113,8 +113,7 @@ std::unique_ptr<AudioLog> MockAudioManager::CreateAudioLog(
   return nullptr;
 }
 
-void MockAudioManager::InitializeOutputDebugRecording(
-    scoped_refptr<base::SingleThreadTaskRunner> file_task_runner) {}
+void MockAudioManager::InitializeOutputDebugRecording() {}
 
 void MockAudioManager::EnableOutputDebugRecording(
     const base::FilePath& base_file_name) {}