Adding AddTrack/RemoveTrack to native PeerConnection API.

Also, now creating the RtpSender/RtpReceiver proxy objects immediately,
rather than waiting until when GetSenders/GetReceivers is called.

Review URL: https://codereview.webrtc.org/1563403002

Cr-Commit-Position: refs/heads/master@{#11259}
diff --git a/talk/app/webrtc/peerconnection.h b/talk/app/webrtc/peerconnection.h
index 6e2b967..03b5853 100644
--- a/talk/app/webrtc/peerconnection.h
+++ b/talk/app/webrtc/peerconnection.h
@@ -83,6 +83,11 @@
   bool AddStream(MediaStreamInterface* local_stream) override;
   void RemoveStream(MediaStreamInterface* local_stream) override;
 
+  rtc::scoped_refptr<RtpSenderInterface> AddTrack(
+      MediaStreamTrackInterface* track,
+      std::vector<MediaStreamInterface*> streams) override;
+  bool RemoveTrack(RtpSenderInterface* sender) override;
+
   virtual WebRtcSession* session() { return session_.get(); }
 
   rtc::scoped_refptr<DtmfSenderInterface> CreateDtmfSender(