Add spread call micro-benchmarks.

test/js-perf-test/SixSpeed/spread/spread__es6.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This benchmark is based on the six-speed benchmark build output.
+// Copyright 2014 Kevin Decker <https://github.com/kpdecker/six-speed/>
+
+"use strict";
+(function(test) {
+  "use strict";
+  function fn() {
+    return Math.max(...[1,2,3]);
+  }
+  test("Spread", "ES6", fn);
+})(benchTest);