Wire scalability_mode when simulcast is not in use (i.e. streams==1)

Bug: webrtc:12148, webrtc:11607
Change-Id: I50047896d1ca610e1a058ad23015e2af2ffe4a26
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/200980
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32925}
diff --git a/media/engine/webrtc_video_engine.cc b/media/engine/webrtc_video_engine.cc
index 7a619f4..84b61db 100644
--- a/media/engine/webrtc_video_engine.cc
+++ b/media/engine/webrtc_video_engine.cc
@@ -3524,7 +3524,7 @@
           *encoder_config.simulcast_layers[0].num_temporal_layers;
     }
   }
-
+  layer.scalability_mode = encoder_config.simulcast_layers[0].scalability_mode;
   layers.push_back(layer);
   return layers;
 }