Ensures that audio tasks cannot run after AudioManager is deleted.

chromecast/media/audio/cast_audio_manager.h

Index: chromecast/media/audio/cast_audio_manager.h
diff --git a/chromecast/media/audio/cast_audio_manager.h b/chromecast/media/audio/cast_audio_manager.h
index 677954eac15303e111e3e871a42ac61c3c78432c..428e845cf438708b8fad989ce381f8b19286f051 100644
--- a/chromecast/media/audio/cast_audio_manager.h
+++ b/chromecast/media/audio/cast_audio_manager.h
@@ -30,6 +30,7 @@ class CastAudioManager : public ::media::AudioManagerBase {
       ::media::AudioLogFactory* audio_log_factory,
       MediaPipelineBackendManager* backend_manager,
       CastAudioMixer* audio_mixer);
+  ~CastAudioManager() override;
 
   // AudioManager implementation.
   bool HasAudioOutputDevices() override;
@@ -48,9 +49,6 @@ class CastAudioManager : public ::media::AudioManagerBase {
   virtual std::unique_ptr<MediaPipelineBackend> CreateMediaPipelineBackend(
       const MediaPipelineDeviceParams& params);
 
- protected:
-  ~CastAudioManager() override;
-
  private:
   // AudioManagerBase implementation.
   ::media::AudioOutputStream* MakeLinearOutputStream(