Implement the NackModule as part of the new jitter buffer.

webrtc/modules/video_coding/include/video_coding_defines.h

Index: webrtc/modules/video_coding/include/video_coding_defines.h
diff --git a/webrtc/modules/video_coding/include/video_coding_defines.h b/webrtc/modules/video_coding/include/video_coding_defines.h
index 7d084c815b8f2ea2a92215bfc0a4394210783688..b70432fcee6d081cc67fb17f49473bbb89a62c47 100644
--- a/webrtc/modules/video_coding/include/video_coding_defines.h
+++ b/webrtc/modules/video_coding/include/video_coding_defines.h
@@ -160,6 +160,8 @@ class VCMFrameTypeCallback {
 // Callback class used for telling the user about which packet sequence numbers
 // are currently
 // missing and need to be resent.
+// TODO(philipel): Deprecate VCMPacketRequestCallback
+//                 and use VCMNackSender instead.
 class VCMPacketRequestCallback {
  public:
   virtual int32_t ResendPackets(const uint16_t* sequenceNumbers,
@@ -169,6 +171,23 @@ class VCMPacketRequestCallback {
   virtual ~VCMPacketRequestCallback() {}
 };
 
+class VCMNackSender {
+ public:
+  virtual int32_t SendNack(const uint16_t* sequence_numbers,
+                           uint16_t length) = 0;
+
+ protected:
+  virtual ~VCMNackSender() {}
+};
+
+class VCMKeyFrameRequestSender {
+ public:
+  virtual int32_t RequestKeyFrame() = 0;
+
+ protected:
+  virtual ~VCMKeyFrameRequestSender() {}
+};
+
 // Callback used to inform the user of the the desired resolution
 // as subscribed by Media Optimization (Quality Modes)
 class VCMQMSettingsCallback {