Add AudioStreamRegistry. Move stream counting logic

media/audio/audio_streams_tracker.h

 // Copyright 2015 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_STREAMS_TRACKER_H_
 #define MEDIA_AUDIO_AUDIO_STREAMS_TRACKER_H_
 
 #include <stddef.h>
 
 #include "base/gtest_prod_util.h"
@@ skipping 11 matching lines @@
   ~AudioStreamsTracker();
 
   // Increases/decreases current stream count. Updates max stream count if
   // current count is larger.
   void IncreaseStreamCount();
   void DecreaseStreamCount(size_t count = 1);
 
   // 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:
   FRIEND_TEST_ALL_PREFIXES(AudioStreamsTrackerTest, IncreaseAndDecreaseOnce);
   FRIEND_TEST_ALL_PREFIXES(AudioStreamsTrackerTest,
                            IncreaseAndDecreaseMultiple);
 
   // Confirms single-threaded access.
   base::ThreadChecker thread_checker_;
 
   // Stores the current and maximum number of streams.
   size_t current_stream_count_;
   size_t max_stream_count_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioStreamsTracker);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_AUDIO_AUDIO_STREAMS_TRACKER_H_