Refactor AudioParameters member setting.

media/audio/mac/audio_manager_mac.cc

Index: media/audio/mac/audio_manager_mac.cc
diff --git a/media/audio/mac/audio_manager_mac.cc b/media/audio/mac/audio_manager_mac.cc
index 42651ffbea8afe2190c75fa36b3b036a5b17c8ce..1bfaf3395147e311e47151e79ba9f226178510a4 100644
--- a/media/audio/mac/audio_manager_mac.cc
+++ b/media/audio/mac/audio_manager_mac.cc
@@ -672,9 +672,11 @@ AudioParameters AudioManagerMac::GetPreferredOutputStreamParameters(
       channel_layout = CHANNEL_LAYOUT_DISCRETE;
   }
 
-  return AudioParameters(
-      AudioParameters::AUDIO_PCM_LOW_LATENCY, channel_layout, output_channels,
-      hardware_sample_rate, 16, buffer_size, AudioParameters::NO_EFFECTS);
+  AudioParameters params(AudioParameters::AUDIO_PCM_LOW_LATENCY, channel_layout,
+                         hardware_sample_rate, 16, buffer_size,
+                         AudioParameters::NO_EFFECTS);
+  params.set_channels_for_discrete(output_channels);
+  return params;
 }
 
 void AudioManagerMac::InitializeOnAudioThread() {