Add cdf_update_mode experiment

Allow the CDF update to operate in different modes, e.g. update vs no
update.

The update mode is transmitted in the uncompressed frame header of
every keyframe and intra-only frame.

This patch only adds bitstream signaling and API support. The
implementation of the update modes will be in later patches.

Change-Id: Ic9fcd60e8a75f9c01f414253823d78cf9b3113dd
diff --git a/av1/encoder/encodetxb.c b/av1/encoder/encodetxb.c
index 83d4598..569b3d2 100644
--- a/av1/encoder/encodetxb.c
+++ b/av1/encoder/encodetxb.c
@@ -209,28 +209,32 @@
       break;
     case 3:
       ++counts->eob_multi128[plane][eob_multi_ctx][eob_pt - 1];
-      if (allow_update_cdf)
+      if (allow_update_cdf) {
         update_cdf(ec_ctx->eob_flag_cdf128[plane][eob_multi_ctx], eob_pt - 1,
                    8);
+      }
       break;
     case 4:
       ++counts->eob_multi256[plane][eob_multi_ctx][eob_pt - 1];
-      if (allow_update_cdf)
+      if (allow_update_cdf) {
         update_cdf(ec_ctx->eob_flag_cdf256[plane][eob_multi_ctx], eob_pt - 1,
                    9);
+      }
       break;
     case 5:
       ++counts->eob_multi512[plane][eob_multi_ctx][eob_pt - 1];
-      if (allow_update_cdf)
+      if (allow_update_cdf) {
         update_cdf(ec_ctx->eob_flag_cdf512[plane][eob_multi_ctx], eob_pt - 1,
                    10);
+      }
       break;
     case 6:
     default:
       ++counts->eob_multi1024[plane][eob_multi_ctx][eob_pt - 1];
-      if (allow_update_cdf)
+      if (allow_update_cdf) {
         update_cdf(ec_ctx->eob_flag_cdf1024[plane][eob_multi_ctx], eob_pt - 1,
                    11);
+      }
       break;
   }