Set marker bit on last encoded spatial layer.

In order to handle per-layer frame dropping both VP9 encoder wrapper
and RTP packetizer were modified.

- Encoder wrapper buffers last encoded frame and passes it to
packetizer after frame of next layer is encoded or encoding of
superframe is finished.
- Encoder wrapper sets end_of_superframe flag on last encoded frame of
superframe before passing it to packetizer.
- If end_of_superframe is True then packetizer sets marker bit on last
packet of frame.

Bug: webrtc:9066
Change-Id: I1d45319fbe6bc63d01721ea67bfb7440d4c29275
Reviewed-on: https://webrtc-review.googlesource.com/65540
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22722}

modules/video_coding/include/video_codec_interface.h

@@ -68,6 +68,8 @@ struct CodecSpecificInfoVP9 {
   // Frame reference data.
   uint8_t num_ref_pics;
   uint8_t p_diff[kMaxVp9RefPics];
+
+  bool end_of_superframe;
 };
 
 struct CodecSpecificInfoGeneric {