Add rtp_timestamp to RtpPacketSendInfo.

We want to make this information available to RtpTransportControllerSend.

Bug: None
Change-Id: Id9237fe1a1fe65834cf2ac79155bc4502744e4db
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/216683
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33914}

modules/rtp_rtcp/include/rtp_rtcp_defines.h

@@ -229,6 +229,7 @@ struct RtpPacketSendInfo {
   uint16_t transport_sequence_number = 0;
   uint32_t ssrc = 0;
   uint16_t rtp_sequence_number = 0;
+  uint32_t rtp_timestamp = 0;
   size_t length = 0;
   absl::optional<RtpPacketMediaType> packet_type;
   PacedPacketInfo pacing_info;