[js-perf-test] Add RegExp microbenchmarks

test/js-perf-test/RegExp/base_flags.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.
+
+var re;
+
+function Global() {
+  var g = re.global;
+}
+
+function Flags() {
+  var f = re.flags;
+}
+
+function Setup() {
+  re = /[Cz]/giym;
+}
+
+var benchmarks = [ [Global, Setup],
+                   [Flags, Setup],
+                 ];