Add RTPVideoHeader const accessor.

Preparation CL to remove RTPTypeHeader.

Follow up to this CL (https://webrtc-review.googlesource.com/c/src/+/84423).

Bug: none
Change-Id: I40516c1791c1ead45e082f554f2f5fcda529e7d6
Reviewed-on: https://webrtc-review.googlesource.com/84588
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23698}

modules/include/module_common_types.h

diff --git a/modules/include/module_common_types.h b/modules/include/module_common_types.h
index b5b023d..c22e995 100644
--- a/modules/include/module_common_types.h
+++ b/modules/include/module_common_types.h
@@ -72,6 +72,7 @@
   FrameType frameType;
   // TODO(philipel): Remove with RTPTypeHeader.
   RTPVideoHeader& video_header() { return type.Video; }
+  const RTPVideoHeader& video_header() const { return type.Video; }
   RTPTypeHeader type;
   // NTP time of the capture time in local timebase in milliseconds.
   int64_t ntp_time_ms;