Adding CastRemotingSender for media remoting.

chrome/renderer/media/cast_transport_ipc.cc

Index: chrome/renderer/media/cast_transport_ipc.cc
diff --git a/chrome/renderer/media/cast_transport_ipc.cc b/chrome/renderer/media/cast_transport_ipc.cc
index b0c1e761e34189325f214e95f3a5a9d0cb82dca0..518610fd3c41775523a5dbf9ca5a9372da43edde 100644
--- a/chrome/renderer/media/cast_transport_ipc.cc
+++ b/chrome/renderer/media/cast_transport_ipc.cc
@@ -41,8 +41,10 @@ CastTransportIPC::~CastTransportIPC() {
 void CastTransportIPC::InitializeStream(
     const media::cast::CastTransportRtpConfig& config,
     std::unique_ptr<media::cast::RtcpObserver> rtcp_observer) {
-  DCHECK(clients_.find(config.ssrc) == clients_.end());
-  clients_[config.ssrc] = std::move(rtcp_observer);
+  if (rtcp_observer) {
+    DCHECK(clients_.find(config.ssrc) == clients_.end());
+    clients_[config.ssrc] = std::move(rtcp_observer);
+  }
   Send(new CastHostMsg_InitializeStream(channel_id_, config));
 }