Add video send SSRC to RtpParameters, and don't allow changing SSRC.

webrtc/media/engine/webrtcvideoengine2.cc

Index: webrtc/media/engine/webrtcvideoengine2.cc
diff --git a/webrtc/media/engine/webrtcvideoengine2.cc b/webrtc/media/engine/webrtcvideoengine2.cc
index 9e36697a6155b2c68ad355d89b892639b12488a5..96c60f93d1e4f09b666c548154566bbcdfaa4f24 100644
--- a/webrtc/media/engine/webrtcvideoengine2.cc
+++ b/webrtc/media/engine/webrtcvideoengine2.cc
@@ -1560,6 +1560,11 @@ WebRtcVideoChannel2::WebRtcVideoSendStream::WebRtcVideoSendStream(
 
   sp.GetPrimarySsrcs(&parameters_.config.rtp.ssrcs);
 
+  // ValidateStreamParams should prevent this from happening.
+  RTC_CHECK(!parameters_.config.rtp.ssrcs.empty());

[pthatcher1, 2016/12/14 19:01:38]

This might happen down the road when we support MID only and no signaled SSRCs. 
It might be easier to just change the code now to be:

if (!parameters_.config.rtp.ssrcs.empty()) {
  rtp_parameters_.encodings[0].ssrc =
rtc::Optional<uint32_t>(parameters_.config.rtp.ssrcs[0]);
}

[Taylor Brandstetter, 2016/12/15 00:04:26]

When that change occurs, we can update this RTC_CHECK and add a unit test for
the newly-supported behavior, but for now I think the check serves its purpose.

+  rtp_parameters_.encodings[0].ssrc =
+      rtc::Optional<uint32_t>(parameters_.config.rtp.ssrcs[0]);
+
   // RTX.
   sp.GetFidSsrcs(parameters_.config.rtp.ssrcs,
                  &parameters_.config.rtp.rtx.ssrcs);
@@ -1862,11 +1867,16 @@ WebRtcVideoChannel2::WebRtcVideoSendStream::GetRtpParameters() const {
 
 bool WebRtcVideoChannel2::WebRtcVideoSendStream::ValidateRtpParameters(
     const webrtc::RtpParameters& rtp_parameters) {
+  RTC_DCHECK_RUN_ON(&thread_checker_);
   if (rtp_parameters.encodings.size() != 1) {
     LOG(LS_ERROR)
         << "Attempted to set RtpParameters without exactly one encoding";
     return false;
   }
+  if (rtp_parameters.encodings[0].ssrc != rtp_parameters_.encodings[0].ssrc) {
+    LOG(LS_ERROR) << "Attempted to set RtpParameters with modified SSRC";

[pthatcher1, 2016/12/14 19:01:38]

You might as well use a for-loop here that will be robust for when we add more
than one encoding.

+    return false;
+  }
   return true;
 }