vp9: Speed 8: Enable skip_encode_sb

Neutral in borg tests.

Some clips show 3-4% speed gain on 2 threads on Pixel.

Change-Id: Ic959f34e44892a854551de6e9a3d9ec819ffed00

vp9/encoder/vp9_speed_features.c

@@ -591,6 +591,7 @@ static void set_rt_speed_feature_framesize_independent(
 
   if (speed >= 8) {
     sf->adaptive_rd_thresh = 4;
+    sf->skip_encode_sb = 1;
     if (!cpi->use_svc) cpi->max_copied_frame = 4;
     if (cpi->row_mt && cpi->oxcf.max_threads > 1)
       sf->adaptive_rd_thresh_row_mt = 1;