vp8: Fix condition for update of last_pred_err_mb

For 1 pass cbr screen-content mode: quantity should
only be updated on delta frames.

Change-Id: I16fc47b2805c7527ab4ff25bd8b5a5bd9c2b8976
diff --git a/vp8/encoder/onyx_if.c b/vp8/encoder/onyx_if.c
index 9aca0f2..07be0d9 100644
--- a/vp8/encoder/onyx_if.c
+++ b/vp8/encoder/onyx_if.c
@@ -3950,7 +3950,9 @@
 
     if (cpi->pass == 0 && cpi->oxcf.end_usage == USAGE_STREAM_FROM_SERVER) {
       if (vp8_drop_encodedframe_overshoot(cpi, Q)) return;
-      cpi->last_pred_err_mb = (int)(cpi->mb.prediction_error / cpi->common.MBs);
+      if (cm->frame_type != KEY_FRAME)
+        cpi->last_pred_err_mb =
+            (int)(cpi->mb.prediction_error / cpi->common.MBs);
     }
 
     cpi->projected_frame_size -= vp8_estimate_entropy_savings(cpi);