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 95abeab77a9c9e6dc6e1a1033b083d9f41535e7a..24454dc188a21f190349ec4d612751b836a23fea 100644
--- a/talk/app/webrtc/webrtcsession.cc
+++ b/talk/app/webrtc/webrtcsession.cc
@@ -1747,7 +1747,6 @@ 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());
   }
 
@@ -1755,7 +1754,6 @@ 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());
   }
 
@@ -1763,7 +1761,6 @@ 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());
   }
 }