Added unit test that verifies that VP9 encoder wrapper correctly sets spatial index in EncodedImage. Bug: webrtc:9721 Change-Id: Id1eb83a423e36273f97659dc76cd5a089a836e6a Reviewed-on: https://webrtc-review.googlesource.com/98003 Reviewed-by: Rasmus Brandt <brandtr@webrtc.org> Commit-Queue: Sergey Silkin <ssilkin@webrtc.org> Cr-Commit-Position: refs/heads/master@{#24575}