[Test] Perform warmup first on SixSpeed super_spread benchmarks.

The graphs don't show any performance bump for
https://codereview.chromium.org/2659623002, which I do see a much better result for
locally. I suspect the function isn't being optimized in turbofan when the
benchmarks run. Maybe this warmup flag will trigger that.

BUG=v8:5895

Review-Url: https://codereview.chromium.org/2664783002
Cr-Commit-Position: refs/heads/master@{#42776}

test/js-perf-test/SixSpeed/super_spread/run.js

@@ -16,7 +16,7 @@ function PrintError(name, error) {
   success = false;
 }
 
-BenchmarkSuite.config.doWarmup = undefined;
+BenchmarkSuite.config.doWarmup = true;
 BenchmarkSuite.config.doDeterministic = undefined;
 
 BenchmarkSuite.RunSuites({NotifyResult: PrintResult, NotifyError: PrintError});