Remove hardcoded kValueSizeBytes values from variable-length header extensions.

Since the RtpStreamId and RepairedRtpStreamId extensions can have variable
length, it makes no sense for them to have a constant valueSize field.
The header length calculation in RtpHeaderExtensionMap needed to be changed
for this because it previously worked with the assumption that all header
types have a constant size. Now it's the caller's job to specify the length
of the extensions that it might use.

BUG=webrtc:7433

Review-Url: https://codereview.webrtc.org/2867713003
Cr-Commit-Position: refs/heads/master@{#18179}

webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.cc

@@ -417,10 +417,6 @@ size_t ModuleRtpRtcpImpl::TimeToSendPadding(
   return rtp_sender_->TimeToSendPadding(bytes, pacing_info);
 }
 
-size_t ModuleRtpRtcpImpl::MaxPayloadSize() const {
-  return rtp_sender_->MaxPayloadSize();
-}
-
 size_t ModuleRtpRtcpImpl::MaxRtpPacketSize() const {
   return rtp_sender_->MaxRtpPacketSize();
 }