Frame marking RTP header extension (PART 1: implement extension)

Bug: webrtc:7765
Change-Id: I23896d121afd6be4bce5ff4deaf736149efebcdb
Reviewed-on: https://webrtc-review.googlesource.com/85200
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24695}

modules/video_coding/frame_object.h

@@ -41,6 +41,7 @@ class RtpFrameObject : public EncodedFrame {
   int64_t RenderTime() const override;
   bool delayed_by_retransmission() const override;
   absl::optional<RTPVideoHeader> GetRtpVideoHeader() const;
+  absl::optional<FrameMarking> GetFrameMarking() const;
 
  private:
   rtc::scoped_refptr<PacketBuffer> packet_buffer_;