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vp9: Fix to quality regression issue for 1 pass.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1217

Issue introduced in https://chromium-review.googlesource.com/#/c/339162/.
The factor on avg_frame_qindex[INTER_FRAME] in that CL, used for q-basis to
set active_best, can cause the QP to decrease too slowly or get stuck
in some cases when max-q=63.

Removing that factor fixes the regression in issue#1217.

Change-Id: I97fe28dca3c2bc2d852b1775f39c62164e032d4f

--- a/vp9/encoder/vp9_ratectrl.c

+++ b/vp9/encoder/vp9_ratectrl.c

@@ -953,11 +953,10 @@

                              FIXED_GF_INTERVAL], cm->bit_depth);

       active_best_quality = VPXMAX(qindex + delta_qindex, rc->best_quality);

     } else {

-      // Use the min of the average Q (with some increase) and

-      // active_worst_quality as basis for active_best.

+      // Use the min of the average Q and active_worst_quality as basis for

+      // active_best.

       if (cm->current_video_frame > 1) {

-        q = VPXMIN(((17 * rc->avg_frame_qindex[INTER_FRAME]) >> 4),

-                    active_worst_quality);

+        q = VPXMIN(rc->avg_frame_qindex[INTER_FRAME], active_worst_quality);

         active_best_quality = inter_minq[q];

       } else {

         active_best_quality = inter_minq[rc->avg_frame_qindex[KEY_FRAME]];