Add a simple, safe gain PostProcessor.

chromecast/media/service/cast_renderer.cc

Index: chromecast/media/service/cast_renderer.cc
diff --git a/chromecast/media/service/cast_renderer.cc b/chromecast/media/service/cast_renderer.cc
index efa97fc07a979082bdd2180549bf4b7197136427..ccc2b3f3ca03fb418ae2003d62d40e0aabbeed66 100644
--- a/chromecast/media/service/cast_renderer.cc
+++ b/chromecast/media/service/cast_renderer.cc
@@ -108,7 +108,9 @@ void CastRenderer::Initialize(::media::MediaResource* media_resource,
   AudioContentType content_type;
   if (audio_device_id_ == kAlarmAudioDeviceId) {
     content_type = AudioContentType::kAlarm;
-  } else if (audio_device_id_ == kTtsAudioDeviceId) {
+  } else if (audio_device_id_ == kTtsAudioDeviceId ||
+             audio_device_id_ ==
+                 ::media::AudioDeviceDescription::kCommunicationsDeviceId) {
     content_type = AudioContentType::kCommunication;
   } else {
     content_type = AudioContentType::kMedia;