Factor out AudioOutputDelegate from AudioRendererHost.

content/browser/browser_main_loop.h

Index: content/browser/browser_main_loop.h
diff --git a/content/browser/browser_main_loop.h b/content/browser/browser_main_loop.h
index 1464de7ced9cb6ae38c160ead17e1a935d9a6474..b3c357de44f5a9bb3c05ddc55308f3112f631ef4 100644
--- a/content/browser/browser_main_loop.h
+++ b/content/browser/browser_main_loop.h
@@ -72,6 +72,7 @@ class ClientNativePixmapFactory;
 #endif
 
 namespace content {
+class AudioDeviceThread;
 class BrowserMainParts;
 class BrowserOnlineStateObserver;
 class BrowserThreadImpl;
@@ -287,7 +288,7 @@ class CONTENT_EXPORT BrowserMainLoop {
   // |user_input_monitor_| has to outlive |audio_manager_|, so declared first.
   std::unique_ptr<media::UserInputMonitor> user_input_monitor_;
   // AudioThread needs to outlive |audio_manager_|.
-  std::unique_ptr<base::Thread> audio_thread_;
+  std::unique_ptr<AudioDeviceThread> audio_thread_;
   media::ScopedAudioManagerPtr audio_manager_;
 
   std::unique_ptr<midi::MidiManager> midi_manager_;