Add RTPVideoHeader accessor.

Preparation CL to remove RTPTypeHeader.

Follow up to this CL (https://webrtc-review.googlesource.com/c/src/+/83985).

Bug: none
Change-Id: I5da83f682bd72aec2f8d50998624de92e3404c58
Reviewed-on: https://webrtc-review.googlesource.com/84423
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23693}
diff --git a/modules/include/module_common_types.h b/modules/include/module_common_types.h
index 6554adb..a1889d4 100644
--- a/modules/include/module_common_types.h
+++ b/modules/include/module_common_types.h
@@ -70,6 +70,8 @@
 struct WebRtcRTPHeader {
   RTPHeader header;
   FrameType frameType;
+  // TODO(philipel): Remove with RTPTypeHeader.
+  RTPVideoHeader& video_header() { return type.Video; }
   RTPTypeHeader type;
   // NTP time of the capture time in local timebase in milliseconds.
   int64_t ntp_time_ms;