Add AudioStreamRegistry. Move stream counting logic

media/audio/audio_streams_tracker.h

Index: media/audio/audio_streams_tracker.h
diff --git a/media/audio/audio_streams_tracker.h b/media/audio/audio_streams_tracker.h
index 1071fd717f363af9467e5c5732df6276f876bc96..9b8b1b33eb2e3e028e33fcc2b4121d59776f51f7 100644
--- a/media/audio/audio_streams_tracker.h
+++ b/media/audio/audio_streams_tracker.h
@@ -29,6 +29,9 @@ class MEDIA_EXPORT AudioStreamsTracker {
   // Resets the max stream count, i.e. sets it to the current stream count.
   void ResetMaxStreamCount();
 
+  // Useful for resetting this between tests with different threading.
+  void DetachFromThreadForTesting();
+
   size_t max_stream_count() const;
 
  private: