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

talk/app/webrtc/mediastreaminterface.h

Index: talk/app/webrtc/mediastreaminterface.h
diff --git a/talk/app/webrtc/mediastreaminterface.h b/talk/app/webrtc/mediastreaminterface.h
index 5911e85e8eff7d8c981dbfd6df2ed958abe16bb7..00782cb2a5eef66d515869733ab7c5b234b9802c 100644
--- a/talk/app/webrtc/mediastreaminterface.h
+++ b/talk/app/webrtc/mediastreaminterface.h
@@ -100,6 +100,9 @@ class MediaStreamTrackInterface : public rtc::RefCountInterface,
     kFailed = 3,  // Track negotiation failed.
   };
 
+  static const char kAudioKind[];
+  static const char kVideoKind[];
+
   virtual std::string kind() const = 0;
   virtual std::string id() const = 0;
   virtual bool enabled() const = 0;