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

talk/app/webrtc/peerconnectioninterface.h

Index: talk/app/webrtc/peerconnectioninterface.h
diff --git a/talk/app/webrtc/peerconnectioninterface.h b/talk/app/webrtc/peerconnectioninterface.h
index b7814e583033ac4f741c8d1b245ed14e5c12c143..723d643784dab7c83ad826c21133af17f095ef0a 100644
--- a/talk/app/webrtc/peerconnectioninterface.h
+++ b/talk/app/webrtc/peerconnectioninterface.h
@@ -333,6 +333,12 @@ class PeerConnectionInterface : public rtc::RefCountInterface {
       AudioTrackInterface* track) = 0;
 
   // TODO(deadbeef): Make these pure virtual once all subclasses implement them.
+  // |kind| must be "audio" or "video".
+  virtual rtc::scoped_refptr<RtpSenderInterface> CreateSender(
+      const std::string& kind) {
+    return rtc::scoped_refptr<RtpSenderInterface>();
+  }
+
   virtual std::vector<rtc::scoped_refptr<RtpSenderInterface>> GetSenders()
       const {
     return std::vector<rtc::scoped_refptr<RtpSenderInterface>>();