Use AudioUnit instead of AudioDevice properties to get channel count.

media/audio/mac/scoped_audio_unit.h

+// 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_MAC_SCOPED_AUDIO_UNIT_H_
+#define MEDIA_AUDIO_MAC_SCOPED_AUDIO_UNIT_H_
+
+#include <AudioUnit/AudioUnit.h>
+#include <CoreAudio/CoreAudio.h>
+
+#include "base/macros.h"
+
+namespace media {
+
+// For whatever reason Apple doesn't have constants defined for these; per the
+// documentation, we use bus 0 for output and bus 1 for input:
+// http://developer.apple.com/library/mac/#technotes/tn2091/_index.html
+enum AUElement : AudioUnitElement { OUTPUT = 0, INPUT = 1 };
+
+// A helper class that ensures AudioUnits are properly disposed of.
+class ScopedAudioUnit {
+ public:
+  // Creates a new AudioUnit and sets its device for |element| to |device|. If
+  // the operation fails, is_valid() will return false and audio_unit() will
+  // return nullptr.
+  ScopedAudioUnit(AudioDeviceID device, AUElement element);
+  ~ScopedAudioUnit();
+
+  bool is_valid() const { return audio_unit_ != nullptr; }
+  AudioUnit audio_unit() const { return audio_unit_; }
+
+ private:
+  AudioUnit audio_unit_ = nullptr;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedAudioUnit);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_AUDIO_MAC_SCOPED_AUDIO_UNIT_H_