DevTools: Add accessibility test for threads pane

This CL adds an axe accessibility test for the threads pane.

Bug: 963183
Change-Id: I35dc2738f7bd5fce2e4b24aaa6a86b51b671b982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707832Reviewed-by: Robert Paveza <Rob.Paveza@microsoft.com>
Commit-Queue: Jack Lynch <jalyn@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#729933}

third_party/blink/web_tests/http/tests/devtools/a11y-axe-core/sources/threads-a11y-test-expected.txt

+Testing accessibility in the threads sidebar pane.
+Threads sidebar pane content:
+ Mainworker-source.jspaused
+Running the axe-core linter on the threads sidebar pane.
+aXe violations: []
+
+

third_party/blink/web_tests/http/tests/devtools/a11y-axe-core/sources/threads-a11y-test.js

+// Copyright 2019 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.
+
+(async function() {
+  TestRunner.addResult('Testing accessibility in the threads sidebar pane.');
+
+  await TestRunner.loadModule('axe_core_test_runner');
+  await TestRunner.loadModule('sources_test_runner');
+  await TestRunner.showPanel('sources');
+  await SourcesTestRunner.startDebuggerTestPromise(/* quiet */ true);
+
+  await TestRunner.evaluateInPagePromise(`new Worker('../../sources/resources/worker-source.js')`);
+  await SourcesTestRunner.waitUntilPausedPromise();
+  const sourcesPanel = UI.panels.sources;
+  sourcesPanel._showThreadsIfNeeded();
+
+  const threadsSidebarPane = await sourcesPanel._threadsSidebarPane.widget();
+  const threadsSidebarElement = threadsSidebarPane.contentElement;
+  TestRunner.addResult(`Threads sidebar pane content:\n ${threadsSidebarElement.deepTextContent()}`);
+  TestRunner.addResult('Running the axe-core linter on the threads sidebar pane.');
+  await AxeCoreTestRunner.runValidation(threadsSidebarElement);
+  TestRunner.completeTest();
+
+})();