[Test] Enable turbofan for spread call SixSpeed tests.

In preparation for more perf work in turbofan, so that we can actually
see the results on the graph.

BUG=v8:5932

Review-Url: https://codereview.chromium.org/2676263002
Cr-Commit-Position: refs/heads/master@{#42960}

test/js-perf-test/SixSpeed.json

@@ -59,6 +59,7 @@
     {
       "name": "Spread",
       "path": ["SixSpeed"],
+      "flags": ["--future"],
       "results_regexp": "^%s\\(Score\\): (.+)$",
       "tests": [
         {