Added is_last_packet_in_frame to match is_first_packet_in_frame.

Today we use |is_first_packet_in_frame| to know when a frame begins and the
|markerBit| to know when it ends, but the markerbit does not actually mark the
end of a frame, it marks the end of a picture.

Bug: webrtc:9361
Change-Id: Icc70e6075590cdc31e875a4eb9d489868adbb67c
Reviewed-on: https://webrtc-review.googlesource.com/100160
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24722}

modules/video_coding/packet.h

@@ -36,6 +36,7 @@ class VCMPacket {
   VideoCodecType codec;
 
   bool is_first_packet_in_frame;
+  bool is_last_packet_in_frame;
   VCMNaluCompleteness completeNALU;  // Default is kNaluIncomplete.
   bool insertStartCode;  // True if a start code should be inserted before this
                          // packet.