vp9: 1 pass vbr mode at speed 5: switch to use mv.search to NSTEP.

Change only affects 1 pass, vbr, speed = 5 (real-time mode).

Some improvement for high motion content.
AvgPSNR/SSIM metrics for ytlive set all up, on average ~2%,
some clips (high motion ones) up 4/5%.

Encoder speed down: on mynintendo_x1.1280_720.y4m: 47fps -> 44fps.

Change-Id: I9e3eaa6392dcb6b5b44ee6f43004f97ba859bc11

vp9/encoder/vp9_speed_features.c

@@ -400,6 +400,8 @@ static void set_rt_speed_feature(VP9_COMP *cpi, SPEED_FEATURES *sf,
     sf->mode_search_skip_flags = FLAG_SKIP_INTRA_DIRMISMATCH;
     sf->tx_size_search_method = is_keyframe ? USE_LARGESTALL : USE_TX_8X8;
     sf->simple_model_rd_from_var = 1;
+    if (cpi->oxcf.rc_mode == VPX_VBR)
+      sf->mv.search_method = NSTEP;
 
     if (!is_keyframe) {
       int i;