Fix target bitrate handling for a single layer VP9 screenshare

For a single layer vp9, the target bitrate was not set correctly. This
may cause a problem for screenshare case, since target bitrate is
respected in that case. If it were less than a min bitrate, the only
spatial layer was permanently disabled.

Bug: webrtc:10257
Change-Id: I0980349adfc2970f810acc51a3e2a31ecbb2bbd2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/125681
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26970}

modules/video_coding/video_codec_initializer.cc

@@ -190,6 +190,7 @@ VideoCodec VideoCodecInitializer::VideoEncoderConfigToVideoCodec(
         if (no_spatial_layering) {
           // Use codec's bitrate limits.
           spatial_layers.back().minBitrate = video_codec.minBitrate;
+          spatial_layers.back().targetBitrate = video_codec.maxBitrate;
           spatial_layers.back().maxBitrate = video_codec.maxBitrate;
         }