benchmark: var to const

PR-URL: https://github.com/nodejs/node/pull/13757
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

benchmark/string_decoder/string-decoder.js

@@ -79,7 +79,7 @@ function main(conf) {
     }
   }
 
-  var nChunks = chunks.length;
+  const nChunks = chunks.length;
 
   bench.start();
   for (i = 0; i < n; ++i) {