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

talk/app/webrtc/audiotrack.cc

 /*
  * libjingle
  * Copyright 2004--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/audiotrack.h"
 
 #include <string>
 
 namespace webrtc {
 
-static const char kAudioTrackKind[] = "audio";
+const char MediaStreamTrackInterface::kAudioKind[] = "audio";
 
 AudioTrack::AudioTrack(const std::string& label,
                        AudioSourceInterface* audio_source)
     : MediaStreamTrack<AudioTrackInterface>(label),
       audio_source_(audio_source) {
 }
 
 std::string AudioTrack::kind() const {
-  return kAudioTrackKind;
+  return kAudioKind;
 }
 
 rtc::scoped_refptr<AudioTrack> AudioTrack::Create(
     const std::string& id, AudioSourceInterface* source) {
   rtc::RefCountedObject<AudioTrack>* track =
       new rtc::RefCountedObject<AudioTrack>(id, source);
   return track;
 }
 
 }  // namespace webrtc