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;

[danilchap, 2016/10/20 15:12:58]

do you plan to remove this friendship later, after renaming class to
UlpfecSender?
e.g. by moving commong part of ProducerFec and FlexfecSender into a different
(base or helper) class.

[brandtr, 2016/10/24 12:52:08]

Yes, this friendship will be removed in the future. But this will happen when
FlexfecSender is connected to PacedSender, together with the corresponding API
change. So there will never be a base class that both ProducerFec and
FlexfecSender derives from. In fact, ULPFEC will eventually be deprecated, and
ProducerFec will be removed.

Btw, is it appropriate to place this friend line at the top of the class
declaration, or should it be placed somewhere else?

+
  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