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.h

@@ -209,8 +209,6 @@ class ModuleRtpRtcpImpl : public RtpRtcp, public RTCPReceiver::ModuleRtpRtcp {
 
   void SetTmmbn(std::vector<rtcp::TmmbItem> bounding_set) override;
 
-  size_t MaxPayloadSize() const override;
-
   size_t MaxRtpPacketSize() const override;
 
   void SetMaxRtpPacketSize(size_t max_packet_size) override;