Remove hardcoded kValueSizeBytes values from variable-length header extensions.

webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc b/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
index b89aefef53e44977f2bda96438bd16cb3b4572af..d5501bf95560bf9e04218041cb82cda2ee39ca15 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_sender_video.cc
@@ -249,7 +249,7 @@ void RTPSenderVideo::GetUlpfecConfig(int* red_payload_type,
 
 size_t RTPSenderVideo::CalculateFecPacketOverhead() const {
   if (flexfec_enabled())
-    return flexfec_sender_->MaxPacketOverhead();
+    return flexfec_sender_->MaxPacketOverhead(RTPSender::FecExtensionSizes());
 
   size_t overhead = 0;
   if (red_enabled()) {
@@ -264,7 +264,8 @@ size_t RTPSenderVideo::CalculateFecPacketOverhead() const {
     // from an FEC viewpoint, they are part of the payload to be protected.
     // (The base RTP header is already protected by the FEC header.)
     overhead += ulpfec_generator_.MaxPacketOverhead() +
-                (rtp_sender_->RtpHeaderLength() - kRtpHeaderSize);
+                (rtp_sender_->RtpHeaderLength(RTPSender::FecExtensionSizes()) -
+                 kRtpHeaderSize);
   }
   return overhead;
 }