Ensures that audio tasks cannot run after AudioManager is deleted.

media/audio/alsa/audio_manager_alsa.h

Index: media/audio/alsa/audio_manager_alsa.h
diff --git a/media/audio/alsa/audio_manager_alsa.h b/media/audio/alsa/audio_manager_alsa.h
index 8dff4678cba003d8496c2d2917563e06cd449bd5..a18bb61466f8e674e2c5665b7572c17440cf20ea 100644
--- a/media/audio/alsa/audio_manager_alsa.h
+++ b/media/audio/alsa/audio_manager_alsa.h
@@ -24,10 +24,12 @@ class MEDIA_EXPORT AudioManagerAlsa : public AudioManagerBase {
       scoped_refptr<base::SingleThreadTaskRunner> task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> worker_task_runner,
       AudioLogFactory* audio_log_factory);
+  ~AudioManagerAlsa() override;
 
   static void ShowLinuxAudioInputSettings();
 
   // Implementation of AudioManager.
+  void Shutdown() override;
   bool HasAudioOutputDevices() override;
   bool HasAudioInputDevices() override;
   void ShowAudioInputSettings() override;
@@ -55,8 +57,6 @@ class MEDIA_EXPORT AudioManagerAlsa : public AudioManagerBase {
       const LogCallback& log_callback) override;
 
  protected:
-  ~AudioManagerAlsa() override;
-
   AudioParameters GetPreferredOutputStreamParameters(
       const std::string& output_device_id,
       const AudioParameters& input_params) override;