Refactorings to send RTCP packets directly via the RtcpPacket callback, with some simplifications enabled by this. NACK now also sent via RtcpPacket.

BUG=webrtc:2450
R=asapersson@webrtc.org

Review URL: https://codereview.webrtc.org/1309833002 .

Cr-Commit-Position: refs/heads/master@{#10888}

webrtc/modules/rtp_rtcp/source/rtcp_packet.h

@@ -116,14 +116,13 @@ class RtcpPacket {
   size_t HeaderLength() const;
 
   static const size_t kHeaderLength = 4;
+  std::vector<RtcpPacket*> appended_packets_;
 
  private:
   bool CreateAndAddAppended(uint8_t* packet,
                             size_t* index,
                             size_t max_length,
                             PacketReadyCallback* callback) const;
-
-  std::vector<RtcpPacket*> appended_packets_;
 };
 
 // TODO(sprang): Move RtcpPacket subclasses out to separate files.