message: Make RTCError::sctp_cause_code const
  Cr-Commit-Position: refs/heads/master@{#31980}
diff: @@ -137,7 +137,7 @@ class RTC_EXPORT RTCError {
  RTCErrorDetailType error_detail() const { return error_detail_; }
  void set_error_detail(RTCErrorDetailType detail) { error_detail_ = detail; }
  - absl::optional<uint16_t> sctp_cause_code() { return sctp_cause_code_; }
  + absl::optional<uint16_t> sctp_cause_code() const { return sctp_cause_code_; }
  void set_sctp_cause_code(uint16_t cause_code) {
  sctp_cause_code_ = cause_code;
  }
  