Don't override active_worst_quality in 2 pass

Commit db5057c introduced a bug in that the active_worst_quality
selected by the 2 pass rate controller was being overridden for key
frames, causing a severe quality loss.

Change-Id: I4865a6fbe3e94e9b4fb9271c7dd68b455d7b371d
diff --git a/vp8/encoder/ratectrl.c b/vp8/encoder/ratectrl.c
index d87591c..313e645 100644
--- a/vp8/encoder/ratectrl.c
+++ b/vp8/encoder/ratectrl.c
@@ -416,7 +416,8 @@
 
     // TODO: if we separate rate targeting from Q targetting, move this.
     // Reset the active worst quality to the baseline value for key frames.
-    cpi->active_worst_quality = cpi->worst_quality;
+    if (cpi->pass != 2)
+        cpi->active_worst_quality = cpi->worst_quality;
 
 #if 0
     {