Add packetization and coding/decoding of feedback message format.

webrtc/modules/rtp_rtcp/source/rtcp_packet.h

Index: webrtc/modules/rtp_rtcp/source/rtcp_packet.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtcp_packet.h b/webrtc/modules/rtp_rtcp/source/rtcp_packet.h
index d212497edde322b73fe252b1b92c0e33af8602b2..c5432de9d6030544f4d6ea91405950d1712cbce6 100644
--- a/webrtc/modules/rtp_rtcp/source/rtcp_packet.h
+++ b/webrtc/modules/rtp_rtcp/source/rtcp_packet.h
@@ -92,18 +92,27 @@ class RtcpPacket {
                            PacketReadyCallback* callback) const;
 
  protected:
-  RtcpPacket() : kHeaderLength(4) {}
+  RtcpPacket() {}
 
   virtual bool Create(uint8_t* packet,
                       size_t* index,
                       size_t max_length,
                       PacketReadyCallback* callback) const = 0;
 
+  void CreateHeader(uint8_t count_or_format,
+                    uint8_t packet_type,
+                    size_t length,
+                    uint8_t* buffer,
+                    size_t* pos) const;
+
   bool OnBufferFull(uint8_t* packet,
                     size_t* index,
                     RtcpPacket::PacketReadyCallback* callback) const;
 
-  const size_t kHeaderLength;
+  virtual size_t BlockLength() const = 0;
+  size_t HeaderLength() const;
+
+  static const size_t kHeaderLength = 4;
 
  private:
   bool CreateAndAddAppended(uint8_t* packet,
@@ -114,6 +123,8 @@ class RtcpPacket {
   std::vector<RtcpPacket*> appended_packets_;
 };
 
+// TODO(sprang): Move RtcpPacket subclasses out to separate files.
+
 class Empty : public RtcpPacket {
  public:
   Empty() : RtcpPacket() {}
@@ -126,6 +137,8 @@ class Empty : public RtcpPacket {
               size_t max_length,
               RtcpPacket::PacketReadyCallback* callback) const override;
 
+  size_t BlockLength() const override;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(Empty);
 };
@@ -651,6 +664,8 @@ class Nack : public RtcpPacket {
               size_t max_length,
               RtcpPacket::PacketReadyCallback* callback) const override;
 
+  size_t BlockLength() const override;
+
  private:
 
   RTCPUtility::RTCPPacketRTPFBNACK nack_;