Make PayloadRouter own the picture id and tl0 pic idx sequences.

It previously owned only the picture id and only in the
WebRTC-VP8-Forced-Fallback-Encoder-v2 experiment.

Moving responsibility to PayloadRouter ensures that  both
picture id and tl0 idx are continuous over codec changes,
as required by the specs for VP8 and VP9 over RTP.

Bug: webrtc:8830
Change-Id: Ie77356dfec6d1e372b6970189e4c3888451920e6
Reviewed-on: https://webrtc-review.googlesource.com/61640
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22448}

modules/video_coding/codecs/vp9/vp9_impl.cc

@@ -646,6 +646,8 @@ void VP9EncoderImpl::PopulateCodecSpecific(CodecSpecificInfo* codec_specific,
     vp9_info->inter_layer_predicted = true;
   }
 
+  vp9_info->first_frame_in_picture = is_first_frame;
+
   if (pkt.data.frame.flags & VPX_FRAME_IS_KEY) {
     frames_since_kf_ = 0;
   }