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-Commit-Position: refs/heads/master@{#9883}
12 files changed