[NORMATIVE-SYNTAX, ext-skip] Adapt skip_mode_cdfs between frames

BUG=aomedia:1361

Change-Id: I8b40ce7183600d6d51e9adeacfb94cf85909601f

av1/common/entropy.c

diff --git a/av1/common/entropy.c b/av1/common/entropy.c
index 52f49c3..c837c27 100644
--- a/av1/common/entropy.c
+++ b/av1/common/entropy.c
@@ -817,4 +817,8 @@
   AVERAGE_TILE_CDFS(compound_index_cdf);
   AVERAGE_TILE_CDFS(comp_group_idx_cdf);
 #endif  // CONFIG_JNT_COMP
+
+#if CONFIG_EXT_SKIP
+  AVERAGE_TILE_CDFS(skip_mode_cdfs)
+#endif
 }