Remove hardcoded kValueSizeBytes values from variable-length header extensions.

webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h

Index: webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
diff --git a/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h b/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
index 11bc9433a45709dd73255d0d0f3c64766488a613..a1227c835e10dedc533f4764913f67c1ac540c87 100644
--- a/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
+++ b/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
@@ -128,6 +128,16 @@ enum RtxMode {
 
 const size_t kRtxHeaderSize = 2;
 
+struct RtpExtensionSize {
+  RTPExtensionType type;
+  uint8_t value_size;
+};
+
+template <typename Extension>
+constexpr RtpExtensionSize CreateExtensionSize() {
+  return {Extension::kId, Extension::kValueSizeBytes};
+}
+
 struct RTCPSenderInfo {
     uint32_t NTPseconds;
     uint32_t NTPfraction;