Reland of Adding the ability to create an RtpSender without a track.

talk/app/webrtc/videotrack.cc

 /*
  * libjingle
  * Copyright 2011 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  *  1. Redistributions of source code must retain the above copyright notice,
  *     this list of conditions and the following disclaimer.
  *  2. Redistributions in binary form must reproduce the above copyright notice,
@@ skipping 13 matching lines @@
  * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #include "talk/app/webrtc/videotrack.h"
 
 #include <string>
 
 namespace webrtc {
 
-static const char kVideoTrackKind[] = "video";
+const char MediaStreamTrackInterface::kVideoKind[] = "video";
 
 VideoTrack::VideoTrack(const std::string& label,
                        VideoSourceInterface* video_source)
     : MediaStreamTrack<VideoTrackInterface>(label),
       video_source_(video_source) {
   if (video_source_)
     video_source_->AddSink(&renderers_);
 }
 
 VideoTrack::~VideoTrack() {
   if (video_source_)
     video_source_->RemoveSink(&renderers_);
 }
 
 std::string VideoTrack::kind() const {
-  return kVideoTrackKind;
+  return kVideoKind;
 }
 
 void VideoTrack::AddRenderer(VideoRendererInterface* renderer) {
   renderers_.AddRenderer(renderer);
 }
 
 void VideoTrack::RemoveRenderer(VideoRendererInterface* renderer) {
   renderers_.RemoveRenderer(renderer);
 }
 
 bool VideoTrack::set_enabled(bool enable) {
   renderers_.SetEnabled(enable);
   return MediaStreamTrack<VideoTrackInterface>::set_enabled(enable);
 }
 
 rtc::scoped_refptr<VideoTrack> VideoTrack::Create(
     const std::string& id, VideoSourceInterface* source) {
   rtc::RefCountedObject<VideoTrack>* track =
       new rtc::RefCountedObject<VideoTrack>(id, source);
   return track;
 }
 
 }  // namespace webrtc