Add support for VP9 configuration through scalability mode.

Bug: webrtc:13960
Change-Id: Ia930647b15f624a4d10d8d335519b69ffdae6636
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/260983
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36919}
diff --git a/modules/video_coding/video_codec_initializer.cc b/modules/video_coding/video_codec_initializer.cc
index 03f7ffe..99eb67c 100644
--- a/modules/video_coding/video_codec_initializer.cc
+++ b/modules/video_coding/video_codec_initializer.cc
@@ -231,6 +231,11 @@
       if (!config.spatial_layers.empty()) {
         // Layering is set explicitly.
         spatial_layers = config.spatial_layers;
+      } else if (scalability_mode.has_value()) {
+        // Layering is set via scalability mode.
+        spatial_layers = GetVp9SvcConfig(video_codec);
+        if (spatial_layers.empty())
+          break;
       } else {
         size_t first_active_layer = 0;
         for (size_t spatial_idx = 0;