DevTools: improve test infrastructure w/ devtools driving the test

third_party/WebKit/LayoutTests/http/tests/inspector/devtools-js/console-clear.js

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+TestRunner.addResult("Tests that console is cleared upon requestClearMessages call.\n");
+
+TestRunner.loadPanels(["console"])
+  .then(() => TestRunner.loadLazyModules(["source_frame"]))
+  .then(test);
+
+function log() {
+  // Fill console.
+  console.log("one");
+  console.log("two");
+  console.log("three");
+}
+
+async function test() {
+  await TestRunner.evaluateInPagePromise(`(${log.toString()})()`);
+  TestRunner.addResult("=== Before clear ===");
+  TestRunner.dumpConsoleMessages();
+
+  Console.ConsoleView.clearConsole();
+  TestRunner.deprecatedRunAfterPendingDispatches(callback);
+  function callback() {
+    TestRunner.addResult("=== After clear ===");
+    TestRunner.dumpConsoleMessages();
+    TestRunner.completeTest();
+  }
+}