Add FlexfecSender.

webrtc/modules/rtp_rtcp/source/producer_fec.h

Index: webrtc/modules/rtp_rtcp/source/producer_fec.h
diff --git a/webrtc/modules/rtp_rtcp/source/producer_fec.h b/webrtc/modules/rtp_rtcp/source/producer_fec.h
index 35bb7dafbfb54125598a75651b5178b91c114ca0..38ddcad7e89a16e305f805b4341c63afddc0caa2 100644
--- a/webrtc/modules/rtp_rtcp/source/producer_fec.h
+++ b/webrtc/modules/rtp_rtcp/source/producer_fec.h
@@ -19,6 +19,8 @@
 
 namespace webrtc {
 
+class FlexfecSenderImpl;
+
 class RedPacket {
  public:
   explicit RedPacket(size_t length);
@@ -40,6 +42,8 @@ class RedPacket {
 };
 
 class ProducerFec {
+  friend class FlexfecSenderImpl;
+
  public:
   ProducerFec();
   ~ProducerFec();
@@ -74,6 +78,8 @@ class ProducerFec {
       size_t rtp_header_length);
 
  private:
+  explicit ProducerFec(std::unique_ptr<ForwardErrorCorrection> fec);
+
   // Overhead is defined as relative to the number of media packets, and not
   // relative to total number of packets. This definition is inherited from the
   // protection factor produced by video_coding module and how the FEC