Add MID sending to FlexfecSender

Bug: webrtc:4050
Change-Id: I1eefd99cca1c02751d3f5a2d3b57625ccb45323f
Reviewed-on: https://webrtc-review.googlesource.com/64321
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22811}

modules/rtp_rtcp/source/rtp_header_extensions.h

@@ -150,6 +150,10 @@ class VideoTimingExtension {
 // Subclasses must defined kId and kUri static constexpr members.
 class BaseRtpStringExtension {
  public:
+  // String RTP header extensions are limited to 16 bytes because it is the
+  // maximum length that can be encoded with one-byte header extensions.
+  static constexpr uint8_t kMaxValueSizeBytes = 16;
+
   static bool Parse(rtc::ArrayView<const uint8_t> data,
                     StringRtpHeaderExtension* str);
   static size_t ValueSize(const StringRtpHeaderExtension& str) {