Revert "Revert "vp9: Avoid encoder loopfilter for non-reference frames.""

Revert "Revert "vp9: Avoid encoder loopfilter for non-reference frames.""

This reverts commit e9b7f98c56b3b9c99a60eb41b83bf8346b3ad25f.

Reason for revert:
Commit d578bdad fixes the issue (encoder/decoder mismatch
in 3TL datarate test) that causes the original revert.

Original change's description:
> Revert "vp9: Avoid encoder loopfilter for non-reference frames."
>
> This reverts commit 863f860bfcf3bdc26eeecb299aa481d0f63d11ac.
>
> This causes encoder / decoder mismatches in various
> VP9/DatarateTestVP9Large.BasicRateTargeting3TemporalLayers tests
>
> BUG=webm:1408
>
> Change-Id: Ic200c39d7ed9c0b0247ef562f5d6f7b2625f7e14
>

TBR=jzern@google.com,marpan@google.com,builds@webmproject.org,jianj@google.com
BUG=webm:1408

Change-Id: Ifeb81460856d1d56482d4e0477a70ee98f8bfaa6