Add FlexfecSender.

webrtc/modules/rtp_rtcp/source/producer_fec.cc

Index: webrtc/modules/rtp_rtcp/source/producer_fec.cc
diff --git a/webrtc/modules/rtp_rtcp/source/producer_fec.cc b/webrtc/modules/rtp_rtcp/source/producer_fec.cc
index f96437698ba4d4085d85fd5656b131105466bee6..b84150ec8a576db6d101735671498c3f464e1102 100644
--- a/webrtc/modules/rtp_rtcp/source/producer_fec.cc
+++ b/webrtc/modules/rtp_rtcp/source/producer_fec.cc
@@ -97,7 +97,10 @@ size_t RedPacket::length() const {
 }
 
 ProducerFec::ProducerFec()
-    : fec_(ForwardErrorCorrection::CreateUlpfec()),
+    : ProducerFec(ForwardErrorCorrection::CreateUlpfec()) {}
+
+ProducerFec::ProducerFec(std::unique_ptr<ForwardErrorCorrection> fec)
+    : fec_(std::move(fec)),
       num_protected_frames_(0),
       min_num_media_packets_(1) {
   memset(&params_, 0, sizeof(params_));
@@ -143,7 +146,7 @@ int ProducerFec::AddRtpPacketAndGenerateFec(const uint8_t* data_buffer,
   bool complete_frame = false;
   const bool marker_bit = (data_buffer[1] & kRtpMarkerBitMask) ? true : false;
   if (media_packets_.size() < kUlpfecMaxMediaPackets) {
-    // Generic FEC can only protect up to |kUlpfecMaxMediaPackets| packets.
+    // Our packet masks can only protect up to |kUlpfecMaxMediaPackets| packets.
     std::unique_ptr<ForwardErrorCorrection::Packet> packet(
         new ForwardErrorCorrection::Packet());
     packet->length = payload_length + rtp_header_length;