Add conversation_key to CastEventProto message.

chromecast/browser/metrics/cast_metrics_service_client.cc

Index: chromecast/browser/metrics/cast_metrics_service_client.cc
diff --git a/chromecast/browser/metrics/cast_metrics_service_client.cc b/chromecast/browser/metrics/cast_metrics_service_client.cc
index 24fdcfcc008a550413610b3c472c2c1c3b750657..43244cc739f2c9b813a00c5729e156617b045fc0 100644
--- a/chromecast/browser/metrics/cast_metrics_service_client.cc
+++ b/chromecast/browser/metrics/cast_metrics_service_client.cc
@@ -166,7 +166,6 @@ bool CastMetricsServiceClient::IsOffTheRecordSessionActive() {
 }
 
 int32_t CastMetricsServiceClient::GetProduct() {
-  // Chromecast currently uses the same product identifier as Chrome.
   return ::metrics::ChromeUserMetricsExtension::CHROME;
 }