Add improved_mv_pred flag in real-time mode

As mentioned in check-in "Improve motion search in real-time mode",
MV prediction calculation causes speed loss for speed 7 and above.
This change added a flag to turn off this calculation for speed>6
in real-time mode.

Change-Id: I9f4ae5a8bf449222d1784b54e7d315fc8347b2d1
diff --git a/vp8/encoder/onyx_int.h b/vp8/encoder/onyx_int.h
index 0289b4a..d6e4942 100644
--- a/vp8/encoder/onyx_int.h
+++ b/vp8/encoder/onyx_int.h
@@ -185,6 +185,7 @@
 
     int use_fastquant_for_pick;
     int no_skip_block4x4_search;
+    int improved_mv_pred;
 
 } SPEED_FEATURES;