Identify keyframe packets for FEC protection (open-webrtc-toolkit#106)

There is code to set different protection levels for keyframes vs.
delta frames, but because this set_is_key_frame was never called,
the distinction was never used.

Also, clear old media packets from a previous frame, which can
happen if a keyframe request comes from the receiver.

modules/rtp_rtcp/source/rtp_sender_video.cc

@@ -715,6 +715,8 @@ bool RTPSenderVideo::SendVideo(
     packet->set_is_key_frame(video_header.frame_type ==
                              VideoFrameType::kVideoFrameKey);
 
+    packet->set_is_key_frame(video_header.frame_type == VideoFrameType::kVideoFrameKey);
+
     // Put packetization finish timestamp into extension.
     if (packet->HasExtension<VideoTimingExtension>()) {
       packet->set_packetization_finish_time(clock_->CurrentTime());

modules/rtp_rtcp/source/ulpfec_generator.cc

@@ -110,6 +110,11 @@ void UlpfecGenerator::AddPacketAndGenerateFec(const RtpPacketToSend& packet) {
   RTC_DCHECK_RUNS_SERIALIZED(&race_checker_);
   RTC_DCHECK(generated_fec_packets_.empty());
 
+  // reset state upon async keyframe request
+  if (keyframe_in_process_ != packet.is_key_frame()) {
+    ResetState();
+  }
+
   {
     MutexLock lock(&mutex_);
     if (pending_params_) {