Use preferred channel layout on macOS instead of total channel count.

media/base/channel_layout.h

 // Copyright (c) 2012 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_BASE_CHANNEL_LAYOUT_H_
 #define MEDIA_BASE_CHANNEL_LAYOUT_H_
 
 #include "media/base/media_export.h"
 
 namespace media {
@@ skipping 111 matching lines @@
   BACK_LEFT,
   BACK_RIGHT,
   LEFT_OF_CENTER,
   RIGHT_OF_CENTER,
   BACK_CENTER,
   SIDE_LEFT,
   SIDE_RIGHT,
   CHANNELS_MAX = SIDE_RIGHT, // Must always equal the largest value ever logged.
 };
 
+// The maximum number of concurrently active channels for all possible layouts.
+// ChannelLayoutToChannelCount() will never return a value higher than this.
+constexpr int kMaxConcurrentChannels = 8;
+
 // Returns the expected channel position in an interleaved stream.  Values of -1
 // mean the channel at that index is not used for that layout.  Values range
 // from 0 to ChannelLayoutToChannelCount(layout) - 1.
 MEDIA_EXPORT int ChannelOrder(ChannelLayout layout, Channels channel);
 
 // Returns the number of channels in a given ChannelLayout.
 MEDIA_EXPORT int ChannelLayoutToChannelCount(ChannelLayout layout);
 
 // Given the number of channels, return the best layout,
 // or return CHANNEL_LAYOUT_UNSUPPORTED if there is no good match.
 MEDIA_EXPORT ChannelLayout GuessChannelLayout(int channels);
 
 // Returns a string representation of the channel layout.
 MEDIA_EXPORT const char* ChannelLayoutToString(ChannelLayout layout);
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_CHANNEL_LAYOUT_H_