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/rtp_rtcp/source/rtp_format_vp9.cc

@@ -576,9 +576,13 @@ bool RtpPacketizerVp9::NextPacket(RtpPacketToSend* packet) {
   if (!WriteHeaderAndPayload(packet_info, packet, packets_.empty())) {
     return false;
   }
-  packet->SetMarker(packets_.empty() &&
-                    (hdr_.spatial_idx == kNoSpatialIdx ||
-                     hdr_.spatial_idx == hdr_.num_spatial_layers - 1));
+
+  // Ensure end_of_superframe is always set on top spatial layer when it is not
+  // dropped.
+  RTC_DCHECK(hdr_.spatial_idx < hdr_.num_spatial_layers - 1 ||
+             hdr_.end_of_superframe);
+
+  packet->SetMarker(packets_.empty() && hdr_.end_of_superframe);
   return true;
 }