Reason for revert:
Causes regression in VP8 simulcast metrics (receive time, encoded frame size, etc) as two excluded streams' decoders request keyframes periodically, which affects metrics of a selected stream.
Original issue's description:
> In full-stack tests: fixed VP8 simulcast to not decode non-selected streams.
>
> BUG=webrtc:7095
>
> Review-Url: https://codereview.webrtc.org/2728553003
> Cr-Commit-Position: refs/heads/master@{#16948}
> Committed: 8dccd67520TBR=sprang@webrtc.org,kjellander@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7095
Review-Url: https://codereview.webrtc.org/2729623005
Cr-Commit-Position: refs/heads/master@{#16967}