Enable arf usage for speed >= 6, 1 pass vbr.

For speed 6 on ytlive set:
On average, speed slowdown ~5%, quality gain ~2%.

Change-Id: Ia18237cc1d52c54d7e2cb3c71f571cf37ef61b44

vp9/encoder/vp9_speed_features.c

@@ -526,6 +526,8 @@ static void set_rt_speed_feature_framesize_independent(
   }
 
   if (speed >= 6) {
+    if (cpi->oxcf.rc_mode == VPX_VBR && cpi->oxcf.lag_in_frames > 0)
+      sf->use_altref_onepass = 1;
     sf->partition_search_type = VAR_BASED_PARTITION;
     // Turn on this to use non-RD key frame coding mode.
     sf->use_nonrd_pick_mode = 1;