Add a reset to rc tracking for dropped frames

VP9/DatarateTestVP9Large.ChangingDropFrameThresh/[34] fails post the
merge of commit#ffa06b37. This commit adds reset of rc tracking info
when frame is dropped, and fixes the causes of the bad interaction
between the tests and the previous commit.

Change-Id: I848acfd9fcb336359662274325190f94aac76eae

vp9/encoder/vp9_ratectrl.c

diff --git a/vp9/encoder/vp9_ratectrl.c b/vp9/encoder/vp9_ratectrl.c
index f1d4774..32acea0 100644
--- a/vp9/encoder/vp9_ratectrl.c
+++ b/vp9/encoder/vp9_ratectrl.c
@@ -1289,6 +1289,8 @@
   cpi->common.last_frame_type = cpi->common.frame_type;
   cpi->rc.frames_since_key++;
   cpi->rc.frames_to_key--;
+  cpi->rc.rc_2_frame = 0;
+  cpi->rc.rc_1_frame = 0;
 }
 
 // Use this macro to turn on/off use of alt-refs in one-pass mode.