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.h b/call/flexfec_receive_stream_impl.h
index c2407cd..c640ac6 100644
--- a/call/flexfec_receive_stream_impl.h
+++ b/call/flexfec_receive_stream_impl.h
@@ -61,12 +61,15 @@
   // ReceiveStream impl.
   void SetRtpExtensions(std::vector<RtpExtension> extensions) override;
   const RtpConfig& rtp_config() const override { return config_.rtp; }
+  uint32_t remote_ssrc() const { return config_.rtp.remote_ssrc; }
 
  private:
   RTC_NO_UNIQUE_ADDRESS SequenceChecker packet_sequence_checker_;
 
-  // Config. Mostly const, header extensions may change.
-  Config config_ RTC_GUARDED_BY(packet_sequence_checker_);
+  // Config. Mostly const, header extensions may change, which is an exception
+  // case that's specifically handled in `SetRtpExtensions`, which must be
+  // called on the `packet_sequence_checker` thread.
+  const Config config_;
 
   // Erasure code interfacing.
   const std::unique_ptr<FlexfecReceiver> receiver_;