[runtime callstats] Helpers for using runtime callstats in mjsunit tests

Change-Id: I1b350c797093454b0b64dca81ed1d28c35d3ada1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2488685
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70674}

test/mjsunit/runtime-callstats-helpers.js

+// Copyright 2020 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.
+
+// Flags: --allow-natives-syntax
+
+// Add --allow-natives-syntax --runtime-call-stats to your test file in order to
+// use this function. You can suppress the extra printout by calling
+// %GetAndResetRuntimeCallStats() at the end of the test.
+function getRuntimeFunctionCallCount(function_name) {
+  const stats = %GetAndResetRuntimeCallStats();
+  const lines = stats.split("\n");
+  for (let i = 3; i < lines.length - 3; ++i) {
+    const line = lines[i];
+    const m = line.match(/(?<name>\S+)\s+\S+\s+\S+\s+(?<count>\S+)/);
+    if (function_name == m.groups.name) {
+      return m.groups.count;
+    }
+  }
+  return 0;
+}