Enabling spatial layers in VP9Impl. Filter layers in the loopback test.

Handling the case when encoder drops only the higher layer.
Added options to screenshare loopback test to discard high temporal or spatial layers (to view the lower layers).

Review URL: https://codereview.webrtc.org/1287643002

Cr-Original-Commit-Position: refs/heads/master@{#9883}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7f6a6fc0b23795cd4f0aacbf707618c1f3d0a878
12 files changed