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

webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc

Index: webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc b/webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc
index 64bd5aa25c07249f1c6e96fa4aea3a75fef6d784..2f5e2e9b1e5e58314a6011e493c9c8bc2dc7a4f3 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_format_vp9.cc
@@ -578,7 +578,9 @@ bool RtpPacketizerVp9::NextPacket(uint8_t* buffer,
   if (!WriteHeaderAndPayload(packet_info, buffer, bytes_to_send)) {
     return false;
   }
-  *last_packet = packets_.empty();
+  *last_packet =
+      packets_.empty() && (hdr_.spatial_idx == kNoSpatialIdx ||
+                           hdr_.spatial_idx == hdr_.num_spatial_layers - 1);
   return true;
 }