Separate reading remote_ssrc from using the rtp_config() getter.

`remote_ssrc` can be considered const while some other state represented
by rtp_config() can not and also is tied to a specific thread.
Separating access to these variables, makes moving things around easier.

Bug: webrtc:11993
Change-Id: I70aa000daab6174a401e01dca163213174e8f284
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261316
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36818}
diff --git a/call/flexfec_receive_stream_impl.cc b/call/flexfec_receive_stream_impl.cc
index a784481..6c8378d 100644
--- a/call/flexfec_receive_stream_impl.cc
+++ b/call/flexfec_receive_stream_impl.cc
@@ -204,7 +204,10 @@
 void FlexfecReceiveStreamImpl::SetRtpExtensions(
     std::vector<RtpExtension> extensions) {
   RTC_DCHECK_RUN_ON(&packet_sequence_checker_);
-  config_.rtp.extensions = std::move(extensions);
+  // TODO(tommi): Remove this cast once header extensions are managed outside
+  // of the config struct.
+  const_cast<std::vector<RtpExtension>&>(config_.rtp.extensions) =
+      std::move(extensions);
 }
 
 }  // namespace webrtc