Wire up FlexfecSender in RTPSender and add unit tests.

webrtc/modules/rtp_rtcp/source/rtp_sender.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_sender.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_sender.cc b/webrtc/modules/rtp_rtcp/source/rtp_sender.cc
index fafbf11bae730376297198206a402aa41a24b8d1..36915c53afeb7fdf9c3a18bf7de53ce4d7f67f84 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_sender.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_sender.cc
@@ -67,6 +67,7 @@ RTPSender::RTPSender(
     Clock* clock,
     Transport* transport,
     RtpPacketSender* paced_sender,
+    FlexfecSender* flexfec_sender,
     TransportSequenceNumberAllocator* sequence_number_allocator,
     TransportFeedbackObserver* transport_feedback_observer,
     BitrateStatisticsObserver* bitrate_callback,
@@ -81,7 +82,7 @@ RTPSender::RTPSender(
       random_(clock_->TimeInMicroseconds()),
       audio_configured_(audio),
       audio_(audio ? new RTPSenderAudio(clock, this) : nullptr),
-      video_(audio ? nullptr : new RTPSenderVideo(clock, this)),
+      video_(audio ? nullptr : new RTPSenderVideo(clock, this, flexfec_sender)),
       paced_sender_(paced_sender),
       transport_sequence_number_allocator_(sequence_number_allocator),
       transport_feedback_observer_(transport_feedback_observer),