Change RTPVideoTypeHeader to absl::variant and move RTPVideoHeader into its own h/cc file.

Bug: none
Change-Id: If28f57c5ae250afbb47c5d20c9854e9a11182642
Reviewed-on: https://webrtc-review.googlesource.com/87561
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23904}

modules/video_coding/codecs/h264/include/h264_globals.h

@@ -16,6 +16,8 @@
 
 #include <string>
 
+#include "rtc_base/checks.h"
+
 namespace webrtc {
 
 // The packetization types that we support: single, aggregated, and fragmented.