message: mark RTPHeader struct RTC_EXPORT
  so it can be used in downstream Chrome tests
  Cr-Commit-Position: refs/heads/main@{#39163}
diff: @@ -148,7 +148,7 @@ struct RTPHeaderExtension {
  enum { kRtpCsrcSize = 15 }; // RFC 3550 page 13
  -struct RTPHeader {
  +struct RTC_EXPORT RTPHeader {
  RTPHeader();
  RTPHeader(const RTPHeader& other);
  RTPHeader& operator=(const RTPHeader& other);
  