message: Change to more idiomatic map erase.
  Tested: JS application with early video.
  Cr-Commit-Position: refs/heads/master@{#31076}
diff: @@ -1486,18 +1486,17 @@ void WebRtcVideoChannel::ResetUnsignaledRecvStream() {
  RTC_LOG(LS_INFO) << "ResetUnsignaledRecvStream.";
  unsignaled_stream_params_ = StreamParams();
  - // Delete any created default streams.
  + // Delete any created default streams. This is needed to avoid SSRC collisions
  + // in Call's RtpDemuxer, in the case that |this| has created a default video
  + // receiver, and then some other WebRtcVideoChannel gets the SSRC signaled
  + // in the corresponding Unified Plan "m=" section.
  auto it = receive_streams_.begin();
  while (it != receive_streams_.end()) {
  - auto delete_it = receive_streams_.end();
  if (it->second->IsDefaultStream()) {
  - delete_it = it;
  - }
  + DeleteReceiveStream(it->second);
  + receive_streams_.erase(it++);
  + } else {
  ++it;
  - if (delete_it != receive_streams_.end()) {
  - DeleteReceiveStream(delete_it->second);
  - // |it| is not invalidated by this erase.
  - receive_streams_.erase(delete_it->first);
  }
  }
  }
  