Enabling spatial layers in VP9Impl. Filter layers in the loopback test.

webrtc/modules/video_coding/main/source/generic_encoder.cc

Index: webrtc/modules/video_coding/main/source/generic_encoder.cc
diff --git a/webrtc/modules/video_coding/main/source/generic_encoder.cc b/webrtc/modules/video_coding/main/source/generic_encoder.cc
index c0925b95b2fb493018731e564b8aa2cad1f110ba..e4408d1f7e7401f80da155124ff41351c7f7a2ee 100644
--- a/webrtc/modules/video_coding/main/source/generic_encoder.cc
+++ b/webrtc/modules/video_coding/main/source/generic_encoder.cc
@@ -55,9 +55,11 @@ void CopyCodecSpecific(const CodecSpecificInfo* info, RTPVideoHeader* rtp) {
           info->codecSpecific.VP9.inter_layer_predicted;
       rtp->codecHeader.VP9.gof_idx = info->codecSpecific.VP9.gof_idx;
 
+      // Packetizer needs to know the number of spatial layers to correctly set
+      // the marker bit, even when the number won't be written in the packet.
+      rtp->codecHeader.VP9.num_spatial_layers =
+          info->codecSpecific.VP9.num_spatial_layers;
       if (info->codecSpecific.VP9.ss_data_available) {
-        rtp->codecHeader.VP9.num_spatial_layers =
-            info->codecSpecific.VP9.num_spatial_layers;
         rtp->codecHeader.VP9.spatial_layer_resolution_present =
             info->codecSpecific.VP9.spatial_layer_resolution_present;
         if (info->codecSpecific.VP9.spatial_layer_resolution_present) {