DevTools: Add axe accessibility test for the watch expressions pane

This adds an axe test for the watch expressions pane and is dependent
on https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1884528

Bug: 963183
Change-Id: Ib43e9494d8ffa1ac825a6824a34b32d9d3095625
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1652369
Commit-Queue: Jack Lynch <jalyn@microsoft.com>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715068}

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

+Adding watch expression.
+Running the axe-core linter on the empty watch pane.
+aXe violations: []
+
+Watch expression text content: 2 + 2 === 5: false
+Running the axe-core linter on the watch expression.
+aXe violations: []
+
+

third_party/blink/web_tests/http/tests/devtools/a11y-axe-core/sources/watch-expression-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() {
+  await TestRunner.loadModule('axe_core_test_runner');
+  await TestRunner.loadModule('elements_test_runner');
+  await TestRunner.loadModule('sources_test_runner');
+  await TestRunner.showPanel('sources');
+  await TestRunner.navigatePromise('../sources/debugger-breakpoints/resources/dom-breakpoints.html');
+
+  await UI.viewManager.showView('sources.watch');
+  TestRunner.addResult('Adding watch expression.');
+  const watchPane = self.runtime.sharedInstance(Sources.WatchExpressionsSidebarPane);
+  watchPane.doUpdate();
+  TestRunner.addResult('Running the axe-core linter on the empty watch pane.');
+  await AxeCoreTestRunner.runValidation(watchPane.contentElement);
+
+  const watchExpression = watchPane._createWatchExpression('2 + 2 === 5');
+  await TestRunner.addSnifferPromise(Sources.WatchExpression.prototype, '_createWatchExpression');
+
+  const watchExpressionElement = watchExpression.treeElement().listItemElement;
+  TestRunner.addResult(`Watch expression text content: ${watchExpressionElement.deepTextContent()}`);
+  TestRunner.addResult('Running the axe-core linter on the watch expression.');
+  await AxeCoreTestRunner.runValidation(watchExpressionElement);
+  TestRunner.completeTest();
+})();