Make AudioDebugFileWriter create its own file worker thread.

content/browser/browser_main_loop.cc

Index: content/browser/browser_main_loop.cc
diff --git a/content/browser/browser_main_loop.cc b/content/browser/browser_main_loop.cc
index 23c5a3187f820242d91e0b8727c58a05a1152487..8c65f5c3dd41a77abf0edb3954f38574020ee6b0 100644
--- a/content/browser/browser_main_loop.cc
+++ b/content/browser/browser_main_loop.cc
@@ -1776,7 +1776,6 @@ void BrowserMainLoop::CreateAudioManager() {
   if (!audio_manager_) {
     audio_manager_ = media::AudioManager::Create(
         base::MakeUnique<media::AudioThreadImpl>(),
-        BrowserThread::GetTaskRunnerForThread(BrowserThread::FILE),
         MediaInternals::GetInstance());
   }
   CHECK(audio_manager_);