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

talk/app/webrtc/audiotrack.cc

Index: talk/app/webrtc/audiotrack.cc
diff --git a/talk/app/webrtc/audiotrack.cc b/talk/app/webrtc/audiotrack.cc
index b0c91296f9e89c326061244f33e5e24705a9469d..63bd87cb4176c0c41e629d66aa45057385fdabc9 100644
--- a/talk/app/webrtc/audiotrack.cc
+++ b/talk/app/webrtc/audiotrack.cc
@@ -31,7 +31,7 @@
 
 namespace webrtc {
 
-static const char kAudioTrackKind[] = "audio";
+const char MediaStreamTrackInterface::kAudioKind[] = "audio";
 
 AudioTrack::AudioTrack(const std::string& label,
                        AudioSourceInterface* audio_source)
@@ -40,7 +40,7 @@ AudioTrack::AudioTrack(const std::string& label,
 }
 
 std::string AudioTrack::kind() const {
-  return kAudioTrackKind;
+  return kAudioKind;
 }
 
 rtc::scoped_refptr<AudioTrack> AudioTrack::Create(