Ensures that audio tasks cannot run after AudioManager is deleted.

media/audio/audio_thread.h

Index: media/audio/audio_thread.h
diff --git a/media/audio/audio_thread.h b/media/audio/audio_thread.h
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+++ b/media/audio/audio_thread.h
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+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MEDIA_AUDIO_AUDIO_THREAD_H_
+#define MEDIA_AUDIO_AUDIO_THREAD_H_
+
+#include "media/base/media_export.h"
+
+namespace base {
+class SingleThreadTaskRunner;
+}  // namespace base
+
+namespace media {
+
+// This class encapulates the logic for the thread and task runners that the
+// AudioManager and related classes run on.
+class MEDIA_EXPORT AudioThread {
+ public:
+  virtual ~AudioThread() {}
+
+  // Synchronously stops all underlying threads.
+  virtual void Stop() = 0;
+
+  // Returns the task runner used for audio IO.
+  // It always returns a non-null task runner (even after Stop has been called).
+  virtual base::SingleThreadTaskRunner* GetTaskRunner() = 0;
+
+  // Heavyweight tasks should use GetWorkerTaskRunner() instead of
+  // GetTaskRunner(). On most platforms they are the same, but some share the
+  // UI loop with the audio IO loop.
+  // It always returns a non-null task runner (even after Stop has been called).
+  virtual base::SingleThreadTaskRunner* GetWorkerTaskRunner() = 0;
+};
+
+}  // namespace media
+
+#endif  // MEDIA_AUDIO_AUDIO_THREAD_H_