Implement the NackModule as part of the new jitter buffer.

Things done/implemented in this CL:
  - An interface that can send Nack (VCMNackSender).
  - An interface that can request KeyFrames (VCMKeyFrameRequestSender).
  - The nack module (NackModule).
  - A set of convenience functions for modular numbers (mod_ops.h).

BUG=webrtc:5514

Review URL: https://codereview.webrtc.org/1715673002

Cr-Commit-Position: refs/heads/master@{#11882}

webrtc/modules/video_coding/include/video_coding_defines.h

@@ -11,6 +11,8 @@
 #ifndef WEBRTC_MODULES_VIDEO_CODING_INCLUDE_VIDEO_CODING_DEFINES_H_
 #define WEBRTC_MODULES_VIDEO_CODING_INCLUDE_VIDEO_CODING_DEFINES_H_
 
+#include <vector>
+
 #include "webrtc/modules/include/module_common_types.h"
 #include "webrtc/typedefs.h"
 #include "webrtc/video_frame.h"
@@ -160,6 +162,8 @@ 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 NackSender instead.
 class VCMPacketRequestCallback {
  public:
   virtual int32_t ResendPackets(const uint16_t* sequenceNumbers,
@@ -169,6 +173,22 @@ class VCMPacketRequestCallback {
   virtual ~VCMPacketRequestCallback() {}
 };
 
+class NackSender {
+ public:
+  virtual void SendNack(const std::vector<uint16_t>& sequence_numbers) = 0;
+
+ protected:
+  virtual ~NackSender() {}
+};
+
+class KeyFrameRequestSender {
+ public:
+  virtual void RequestKeyFrame() = 0;
+
+ protected:
+  virtual ~KeyFrameRequestSender() {}
+};
+
 // Callback used to inform the user of the the desired resolution
 // as subscribed by Media Optimization (Quality Modes)
 class VCMQMSettingsCallback {