VideoStreamEncoder report VideoLayersAllocation for simulcast

Adds support for Vp8 simulcast.

Bug: webrtc:12000
Change-Id: Ib24fd0542642b023ec35f7a7bdc4880d72365edf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187341
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32416}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 115c71a..c6774dc 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -226,6 +226,7 @@
     "../api/video:video_bitrate_allocator_factory",
     "../api/video:video_codec_constants",
     "../api/video:video_frame",
+    "../api/video:video_layers_allocation",
     "../api/video:video_rtp_headers",
     "../api/video:video_stream_encoder",
     "../api/video_codecs:video_codecs_api",