Unfortunate typo and weak tests made it so if only a middle spatial layer
is active, vp9 encoder would be configured to send two top layers.
Bug: webrtc:11319
Change-Id: I460c245044f60ea7e0127c0e4134d0edab85f4f3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185043
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32164}