[js-perf-test] Add String#{starts,ends}With micro-benchmark

This patch adds a micro-benchmark comparing
`string.startsWith(singleCodeUnit)`, `string[0]`,
`string.endsWith(singleCodeUnit)`, and `string[string.length - 1]`.

The benchmark can be used to measure any String#{starts,ends}With
optimizations we implement in the future.

Test:

    tools/run_perf.py --binary-override-path=out/x64.release/d8 \
      --filter=JSTests/Strings/StringStartsEndsWithComparison \
      --extra-flags=--trace-turbo test/js-perf-test/JSTests.json

Bug: v8:7453
Change-Id: I68cad197fbcbfc6b1938fc437776c319ee9f81df
Reviewed-on: https://chromium-review.googlesource.com/1011619Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52589}

test/js-perf-test/JSTests.json

@@ -292,6 +292,20 @@
             {"name": "StringMatchAllBuiltinZeroWidthUnicode"}
           ]
         },
+        {
+          "name": "StringStartsEndsWithComparison",
+          "main": "run.js",
+          "resources": [ "string-startsendswith-comp.js" ],
+          "test_flags": [ "string-startsendswith-comp" ],
+          "results_regexp": "^%s\\-Strings\\(Score\\): (.+)$",
+          "run_count": 1,
+          "tests": [
+            {"name": "startsWith"},
+            {"name": "startsIndex"},
+            {"name": "endsWith"},
+            {"name": "endsIndex"}
+          ]
+        },
         {
           "name": "StringSubstring",
           "main": "run.js",

test/js-perf-test/Strings/string-startsendswith-comp.js

+// Copyright 2018 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.
+
+function benchy(fn, name) {
+  new BenchmarkSuite(name, [1], [
+    new Benchmark(name, true, false, 0, fn),
+  ]);
+}
+
+const inputs = [
+  'I\xF1t\xEBrn\xE2ti\xF4n\xE0liz\xE6ti\xF8n\u2603\uD83D\uDCA9\uFFFD',
+  'Lorem ipsum dolor sit amet, consectetur adipiscing elit.',
+  'Integer eu augue suscipit, accumsan ipsum nec, sagittis sem.',
+  'In vitae pellentesque dolor. Curabitur leo nunc, luctus vitae',
+  'risus eget, fermentum hendrerit justo.',
+];
+const first = 'I';
+
+function helper(fn) {
+  let sum = 0;
+  for (const input of inputs) {
+    sum += fn(input);
+  }
+  return sum;
+}
+
+function startsWith(string) {
+  return string.startsWith(first);
+}
+
+function startsIndex(string) {
+  return string[0] === first;
+}
+
+function endsWith(string) {
+  return string.endsWith(first);
+}
+
+function endsIndex(string) {
+  return string[string.length - 1] === first;
+}
+
+benchy(() => helper(startsWith), 'startsWith');
+benchy(() => helper(startsIndex), 'startsIndex');
+benchy(() => helper(endsWith), 'endsWith');
+benchy(() => helper(endsIndex), 'endsIndex');