Reenable the dcheck ssrc can't change after first sent packet

Bug: webrtc:6887
Change-Id: I6eb1ffc7dd98390f870b15132ba6038dd6c57b87
Reviewed-on: https://webrtc-review.googlesource.com/36301
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21453}
diff --git a/modules/rtp_rtcp/source/rtp_sender.cc b/modules/rtp_rtcp/source/rtp_sender.cc
index bc88039..9ee5f3b 100644
--- a/modules/rtp_rtcp/source/rtp_sender.cc
+++ b/modules/rtp_rtcp/source/rtp_sender.cc
@@ -927,10 +927,7 @@
   // To support retransmissions, we store the media packet as sent in the
   // packet history (even if send failed).
   if (storage == kAllowRetransmission) {
-    // TODO(brandtr): Uncomment the DCHECK line below when |ssrc_| cannot
-    // change after the first packet has been sent. For more details, see
-    // https://bugs.chromium.org/p/webrtc/issues/detail?id=6887.
-    // RTC_DCHECK_EQ(ssrc, SSRC());
+    RTC_DCHECK_EQ(ssrc, SSRC());
     packet_history_.PutRtpPacket(std::move(packet), storage, true);
   }