Fix unit test on droppable frames

Skip arf stack update for droppable frames.

Change-Id: Ie1d5da16ac2ed045b28f5a9ffb20a8369c2c9a7b
diff --git a/av1/encoder/encode_strategy.c b/av1/encoder/encode_strategy.c
index fad2562..1912b61 100644
--- a/av1/encoder/encode_strategy.c
+++ b/av1/encoder/encode_strategy.c
@@ -823,6 +823,8 @@
   // expressed than converting the frame update type.
   if (frame_is_sframe(cm)) frame_update_type = KEY_FRAME;
 
+  if (is_frame_droppable(cpi)) return;
+
   switch (frame_update_type) {
     case KEY_FRAME:
       stack_reset(ref_buffer_stack->lst_stack,
@@ -1029,6 +1031,8 @@
     return 0;
   }
 
+  if (is_frame_droppable(cpi)) return 0;
+
   int refresh_mask = 0;
 
   if (cpi->ext_refresh_frame_flags_pending) {