Adding the ability to create an RtpSender without a track.

talk/app/webrtc/webrtcsession.cc

Index: talk/app/webrtc/webrtcsession.cc
diff --git a/talk/app/webrtc/webrtcsession.cc b/talk/app/webrtc/webrtcsession.cc
index af1dc61a93b184f8d9f26a8615aafb6fc2e077a3..bd547063b985880c9d092d4950a7b44313c9609f 100644
--- a/talk/app/webrtc/webrtcsession.cc
+++ b/talk/app/webrtc/webrtcsession.cc
@@ -1763,7 +1763,6 @@ void WebRtcSession::RemoveUnusedChannels(const SessionDescription* desc) {
       cricket::GetFirstVideoContent(desc);
   if ((!video_info || video_info->rejected) && video_channel_) {
     SignalVideoChannelDestroyed();
-    const std::string content_name = video_channel_->content_name();
     channel_manager_->DestroyVideoChannel(video_channel_.release());
   }
 
@@ -1771,7 +1770,6 @@ void WebRtcSession::RemoveUnusedChannels(const SessionDescription* desc) {
       cricket::GetFirstAudioContent(desc);
   if ((!voice_info || voice_info->rejected) && voice_channel_) {
     SignalVoiceChannelDestroyed();
-    const std::string content_name = voice_channel_->content_name();
     channel_manager_->DestroyVoiceChannel(voice_channel_.release());
   }
 
@@ -1779,7 +1777,6 @@ void WebRtcSession::RemoveUnusedChannels(const SessionDescription* desc) {
       cricket::GetFirstDataContent(desc);
   if ((!data_info || data_info->rejected) && data_channel_) {
     SignalDataChannelDestroyed();
-    const std::string content_name = data_channel_->content_name();
     channel_manager_->DestroyDataChannel(data_channel_.release());
   }
 }