Use JSON_NAME, not the "histogram_hb_v2" constant

sql/opt_histogram_json.cc

@@ -285,7 +285,7 @@ bool Histogram_json_hb::parse(MEM_ROOT *mem_root, Field *field,
   const char *hist_array;
   int hist_array_len;
   if (JSV_ARRAY != json_get_object_key(obj1, obj1 + obj1_len,
-                                       "histogram_hb_v2", &hist_array,
+                                       JSON_NAME, &hist_array,
                                        &hist_array_len))
   {
     err_pos= obj1;

sql/opt_histogram_json.h

@@ -31,8 +31,8 @@
     ]
   }
 
-  The histogram is an object with single member named "histogram_hb_v2".
-  The value of that member is an array of buckets.
+  The histogram is an object with single member named Histogram_json_hb::
+  JSON_NAME. The value of that member is an array of buckets.
   Each bucket is an object with these members:
     "start" - the first value in the bucket.
     "size"  - fraction of table rows that is contained in the bucket.