Initialize the rtp_stream_id field added by https://codereview.chromium.org/2307653002/.

Review-Url: https://codereview.chromium.org/2344813004
Cr-Commit-Position: refs/heads/master@{#419012}

media/cast/net/cast_transport_config.cc

@@ -8,7 +8,10 @@ namespace media {
 namespace cast {
 
 CastTransportRtpConfig::CastTransportRtpConfig()
-    : ssrc(0), feedback_ssrc(0), rtp_payload_type(RtpPayloadType::UNKNOWN) {}
+    : rtp_stream_id(0),
+      ssrc(0),
+      feedback_ssrc(0),
+      rtp_payload_type(RtpPayloadType::UNKNOWN) {}
 
 CastTransportRtpConfig::~CastTransportRtpConfig() {}