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/BUILD.gn

@@ -179,6 +179,7 @@ rtc_static_library("video_coding") {
     "../rtp_rtcp:rtp_rtcp_format",
     "../utility:utility",
     "//third_party/abseil-cpp/absl/types:optional",
+    "//third_party/abseil-cpp/absl/types:variant",
   ]
 }
 
@@ -221,6 +222,10 @@ rtc_source_set("codec_globals_headers") {
     "codecs/vp8/include/vp8_globals.h",
     "codecs/vp9/include/vp9_globals.h",
   ]
+
+  deps = [
+    "../../rtc_base:checks",
+  ]
 }
 
 rtc_source_set("video_coding_utility") {