Add basic building blocks for 8x8 superblocks experiment.

Change-Id: I274a1d2e461e6ffdb106bac4ad6951692ace314e

configure

@@ -250,6 +250,7 @@ EXPERIMENT_LIST="
     sbsegment
     multiple_arf
     code_zerogroup
+    sb8x8
 "
 CONFIG_LIST="
     external_build

vp9/common/vp9_enums.h

@@ -14,6 +14,13 @@
 #include "./vpx_config.h"
 
 typedef enum BLOCK_SIZE_TYPE {
+#if CONFIG_SB8X8
+  BLOCK_SIZE_SB8X8,
+#if CONFIG_SBSEGMENT
+  BLOCK_SIZE_SB8X16,
+  BLOCK_SIZE_SB16X8,
+#endif
+#endif
   BLOCK_SIZE_MB16X16,
 #if CONFIG_SBSEGMENT
   BLOCK_SIZE_SB16X32,