Pass picture_id from generic packetizer through codec-specific field

To free up RtpVideoHeader::generic field for codec agnostic details
from an rtp header extension.

Bug: webrtc:10342
Change-Id: I7b9d869b2ecfedb96dfd860be47ed8dffa058749
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/166175
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30396}

modules/rtp_rtcp/source/rtp_video_header.h

@@ -28,10 +28,18 @@
 #include "modules/video_coding/codecs/vp9/include/vp9_globals.h"
 
 namespace webrtc {
+// Details passed in the rtp payload for legacy generic rtp packetizer.
+// TODO(bugs.webrtc.org/9772): Deprecate in favor of passing generic video
+// details in an rtp header extension.
+struct RTPVideoHeaderLegacyGeneric {
+  uint16_t picture_id;
+};
+
 using RTPVideoTypeHeader = absl::variant<absl::monostate,
                                          RTPVideoHeaderVP8,
                                          RTPVideoHeaderVP9,
-                                         RTPVideoHeaderH264>;
+                                         RTPVideoHeaderH264,
+                                         RTPVideoHeaderLegacyGeneric>;
 
 struct RTPVideoHeader {
   struct GenericDescriptorInfo {