EC_MULTISYMBOL: make all CDFs have a extra element.

This will make it easier to add native CDFs for all the
dependent experiments without excessive macros.

Change-Id: Iee4710f0fe1c1b4300f686cdf2c5b879a36de987
diff --git a/aom_dsp/prob.h b/aom_dsp/prob.h
index c98586c..521a20e 100644
--- a/aom_dsp/prob.h
+++ b/aom_dsp/prob.h
@@ -27,6 +27,10 @@
 // TODO(negge): Rename this aom_prob once we remove vpxbool.
 typedef uint16_t aom_cdf_prob;
 
+#if CONFIG_EC_MULTISYMBOL
+#define CDF_SIZE(x) ((x) + 1)
+#endif
+
 #define CDF_PROB_BITS 15
 #define CDF_PROB_TOP (1 << CDF_PROB_BITS)