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/codecs/vp9/vp9_impl.h

@@ -94,6 +94,8 @@ class VP9EncoderImpl : public VP9Encoder {
   static void EncoderOutputCodedPacketCallback(vpx_codec_cx_pkt* pkt,
                                                void* user_data);
 
+  void DeliverBufferedFrame(bool end_of_superframe);
+
   // Determine maximum target for Intra frames
   //
   // Input:
@@ -103,6 +105,7 @@ class VP9EncoderImpl : public VP9Encoder {
   uint32_t MaxIntraTarget(uint32_t optimal_buffer_size);
 
   EncodedImage encoded_image_;
+  CodecSpecificInfo codec_specific_;
   EncodedImageCallback* encoded_complete_callback_;
   VideoCodec codec_;
   bool inited_;