Initialize channel_layout in CastAudioOutputStream

CastAudioOutputStream::CmaWrapper failed to set the correct value
on |channel_layout| in |audio_config|. The default value is considered
to be invalid and causes error when sent through Mojo pipe.

Bug: internal b/129642940
Test: None
Change-Id: Ie449ee966c4d79717a4951e18b1953f62a109929
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562089Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Guohui Deng <guohuideng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649669}

chromecast/media/audio/cast_audio_output_stream.cc

@@ -207,10 +207,13 @@ void CastAudioOutputStream::CmaWrapper::Initialize(
 
   AudioConfig audio_config;
   audio_config.codec = kCodecPCM;
+  audio_config.channel_layout =
+      ChannelLayoutFromChannelNumber(audio_params_.channels());
   audio_config.sample_format = kSampleFormatS16;
   audio_config.bytes_per_channel = 2;
   audio_config.channel_number = audio_params_.channels();
   audio_config.samples_per_second = audio_params_.sample_rate();
+  DCHECK(IsValidConfig(audio_config));
   if (!audio_decoder_->SetConfig(audio_config)) {
     encountered_error_ = true;
     return;