Add frequency smoothing to postfilter.

Reduces musical noise with minimal impact on interferer suppression.

This also unifies the treatment of "mean bins". The "end" bin is now
inclusive in the mean range as with the "start" bin.

Corrects interpretation of quantile.

BUG=chromium:490477
R=aluebs@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/50939004

Cr-Commit-Position: refs/heads/master@{#9317}
2 files changed