[Chromecast] Implement new volume control API

chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc

Index: chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc
diff --git a/chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc b/chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc
index ed95959b5452f4771c33a56848b0b27bcb17088d..118d6db9984f7c559f12b827fe87956aa42d49d9 100644
--- a/chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc
+++ b/chromecast/media/cma/backend/alsa/media_pipeline_backend_alsa.cc
@@ -102,6 +102,10 @@ std::string MediaPipelineBackendAlsa::DeviceId() const {
   return params_.device_id;
 }
 
+AudioContentType MediaPipelineBackendAlsa::ContentType() const {
+  return params_.content_type;
+}
+
 const scoped_refptr<base::SingleThreadTaskRunner>&
 MediaPipelineBackendAlsa::GetTaskRunner() const {
   return static_cast<TaskRunnerImpl*>(params_.task_runner)->runner();