Experiment for the nack module.

Testing the nack module by implementing it into the current jitter buffer
under the experiment WebRTC-NewVideoJitterBuffer.

BUG=webrtc:5514

Review URL: https://codereview.webrtc.org/1778503002

Cr-Commit-Position: refs/heads/master@{#11969}

webrtc/modules/video_coding/video_coding_impl.h

@@ -129,7 +129,10 @@ class VideoReceiver {
  public:
   typedef VideoCodingModule::ReceiverRobustness ReceiverRobustness;
 
-  VideoReceiver(Clock* clock, EventFactory* event_factory);
+  VideoReceiver(Clock* clock,
+                EventFactory* event_factory,
+                NackSender* nack_sender = nullptr,
+                KeyFrameRequestSender* keyframe_request_sender = nullptr);
   ~VideoReceiver();
 
   int32_t RegisterReceiveCodec(const VideoCodec* receiveCodec,