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

webrtc/video/loopback.h

Index: webrtc/video/loopback.h
diff --git a/webrtc/video/loopback.h b/webrtc/video/loopback.h
index 0a8bb9793bbf66599857b24db92f1b5572ef378a..c671de4e980b6e1c401ab24a69823319ecd3376d 100644
--- a/webrtc/video/loopback.h
+++ b/webrtc/video/loopback.h
@@ -33,6 +33,11 @@ class Loopback {
     int32_t min_transmit_bitrate_kbps;
     std::string codec;
     size_t num_temporal_layers;
+    size_t num_spatial_layers;
+    // Discard all temporal/spatial layers with id greater or equal the
+    // threshold. 0 to disable.
+    size_t tl_discard_threshold;
+    size_t sl_discard_threshold;
     int32_t loss_percent;
     int32_t link_capacity_kbps;
     int32_t queue_size;