Wire up FlexfecSender in RTPSender and add unit tests.

webrtc/modules/rtp_rtcp/include/rtp_rtcp.h

Index: webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
diff --git a/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h b/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
index fe88b1b674c90fdacba76702b9de3fc97d7715c9..9c00b78a9c60b310b5c6fd88e6be4bb245f5ccba 100644
--- a/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
+++ b/webrtc/modules/rtp_rtcp/include/rtp_rtcp.h
@@ -19,6 +19,7 @@
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/base/deprecation.h"
 #include "webrtc/modules/include/module.h"
+#include "webrtc/modules/rtp_rtcp/include/flexfec_sender.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "webrtc/modules/video_coding/include/video_coding_defines.h"
 
@@ -75,6 +76,10 @@ class RtpRtcp : public Module {
     // Spread any bursts of packets into smaller bursts to minimize packet loss.
     RtpPacketSender* paced_sender = nullptr;
 
+    // Generate FlexFEC packets.
+    // TODO(brandtr): Remove when FlexfecSender is wired up to PacedSender.
+    FlexfecSender* flexfec_sender = nullptr;
+
     TransportSequenceNumberAllocator* transport_sequence_number_allocator =
         nullptr;
     BitrateStatisticsObserver* send_bitrate_observer = nullptr;