Fix VPX_KF_DISABLED.

VP9 encoder still inserts key frame periodically when VPX_KF_DISABLED is
set in non SVC for 1-pass CBR.
BUG=webm:1592

Change-Id: Ie99d7c5b95230d739e263a2d87879693c53f620e
diff --git a/vp9/encoder/vp9_ratectrl.c b/vp9/encoder/vp9_ratectrl.c
index 9df2eb3..e342250 100644
--- a/vp9/encoder/vp9_ratectrl.c
+++ b/vp9/encoder/vp9_ratectrl.c
@@ -2271,7 +2271,7 @@
   RATE_CONTROL *const rc = &cpi->rc;
   int target;
   if ((cm->current_video_frame == 0) || (cpi->frame_flags & FRAMEFLAGS_KEY) ||
-      rc->frames_to_key == 0) {
+      (cpi->oxcf.auto_key && rc->frames_to_key == 0)) {
     cm->frame_type = KEY_FRAME;
     rc->frames_to_key = cpi->oxcf.key_freq;
     rc->kf_boost = DEFAULT_KF_BOOST;