Move implmentation specific constants out of rtp_header_extension.h

webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc b/webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc
index 67e62c272ce5b19bb62fcb08139eadff3e23ec6a..bbbb1438dc8bc1d2f0aa6b0fe204b066ef66b6b4 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_header_extension.cc
@@ -96,6 +96,7 @@ bool RtpHeaderExtensionMap::RegisterByUri(uint8_t id, const std::string& uri) {
 }
 
 size_t RtpHeaderExtensionMap::GetTotalLengthInBytes() const {
+  static constexpr size_t kRtpOneByteHeaderLength = 4;
   if (total_values_size_bytes_ == 0)
     return 0;
   return Word32Align(kRtpOneByteHeaderLength + total_values_size_bytes_);