Make FlexFEC packets paceable through RTPSender.

webrtc/modules/rtp_rtcp/source/flexfec_sender_unittest.cc

Index: webrtc/modules/rtp_rtcp/source/flexfec_sender_unittest.cc
diff --git a/webrtc/modules/rtp_rtcp/source/flexfec_sender_unittest.cc b/webrtc/modules/rtp_rtcp/source/flexfec_sender_unittest.cc
index ca114abeb3d4675b3cb5c8613fe572134b983d0f..62f03e8febb8eb48ce77bbd2017c20dfbc6baf54 100644
--- a/webrtc/modules/rtp_rtcp/source/flexfec_sender_unittest.cc
+++ b/webrtc/modules/rtp_rtcp/source/flexfec_sender_unittest.cc
@@ -70,6 +70,14 @@ std::unique_ptr<RtpPacketToSend> GenerateSingleFlexfecPacket(
 
 }  // namespace
 
+TEST(FlexfecSenderTest, Ssrc) {
+  SimulatedClock clock(kInitialSimulatedClockTime);
+  FlexfecSender sender(kFlexfecPayloadType, kFlexfecSsrc, kMediaSsrc,
+                       kNoRtpHeaderExtensions, &clock);
+
+  EXPECT_EQ(kFlexfecSsrc, sender.Ssrc());
+}
+
 TEST(FlexfecSenderTest, NoFecAvailableBeforeMediaAdded) {
   SimulatedClock clock(kInitialSimulatedClockTime);
   FlexfecSender sender(kFlexfecPayloadType, kFlexfecSsrc, kMediaSsrc,