Add AudioStreamRegistry. Move stream counting logic

content/browser/renderer_host/media/audio_stream_registry.h

+// Copyright 2016 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 CONTENT_BROWSER_RENDERER_HOST_MEDIA_AUDIO_STREAM_REGISTRY_H_
+#define CONTENT_BROWSER_RENDERER_HOST_MEDIA_AUDIO_STREAM_REGISTRY_H_
+
+namespace content {
+
+// This class tracks all the output streams and enables stream counting.
+// Methods may only be called on the IO thread.
+class AudioStreamRegistry {
+ public:
+  virtual void RegisterStream() = 0;
+  virtual void DeregisterStream() = 0;
+  virtual void StreamStateChanged(bool playing) = 0;
+
+ protected:
+  virtual ~AudioStreamRegistry() {}
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_RENDERER_HOST_MEDIA_AUDIO_STREAM_REGISTRY_H_