DevTools: Add Scope pane axe test

This CL adds an axe accessibility test for the Sources panel Scope pane.

Bug: 963183
Change-Id: Icb24c0ae8312d38fe26eaa78903472128a01e7cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1668238Reviewed-by: Robert Paveza <Rob.Paveza@microsoft.com>
Commit-Queue: Jack Lynch <jalyn@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#728672}

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

+Testing accessibility in the scope pane.
+
+Script execution paused.
+Scope pane content: Localthis: WindowClosure (makeClosure)WindowGlobal
+Running the axe-core linter on the scope pane.
+aXe violations: []
+
+Expanding the makeClosure closure.
+Scope pane content: Localthis: WindowClosure (makeClosure)x: 5WindowGlobal
+Running the axe-core linter on the scope pane.
+aXe violations: []
+
+Script execution resumed.
+

third_party/blink/web_tests/http/tests/devtools/a11y-axe-core/sources/scope-pane-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('sources_test_runner');
+  await TestRunner.showPanel('sources');
+  await UI.viewManager.showView('sources.scopeChain');
+
+  TestRunner.addResult('Testing accessibility in the scope pane.\n');
+  await SourcesTestRunner.startDebuggerTestPromise();
+  TestRunner.evaluateInPagePromise(`
+  function makeClosure() {
+    let x = 5;
+    function logX() {
+      console.log(x);
+      debugger;
+    }
+    return logX;
+  }
+  makeClosure()();
+  `);
+  await SourcesTestRunner.waitUntilPausedPromise();
+
+  await TestRunner.addSnifferPromise(Sources.ScopeChainSidebarPane.prototype, '_sidebarPaneUpdatedForTest');
+  const scopePane = self.runtime.sharedInstance(Sources.ScopeChainSidebarPane);
+  await TestRunner.addSnifferPromise(ObjectUI.ObjectPropertyTreeElement, 'populateWithProperties');
+  TestRunner.addResult(`Scope pane content: ${scopePane.contentElement.deepTextContent()}`);
+  TestRunner.addResult(`Running the axe-core linter on the scope pane.`);
+  await AxeCoreTestRunner.runValidation(scopePane.contentElement);
+
+  TestRunner.addResult('Expanding the makeClosure closure.');
+  scopePane._treeOutline._rootElement.childAt(1).expand();
+  await TestRunner.addSnifferPromise(ObjectUI.ObjectPropertyTreeElement, 'populateWithProperties');
+  TestRunner.addResult(`Scope pane content: ${scopePane.contentElement.deepTextContent()}`);
+  TestRunner.addResult(`Running the axe-core linter on the scope pane.`);
+  await AxeCoreTestRunner.runValidation(scopePane.contentElement);
+
+  SourcesTestRunner.completeDebuggerTest();
+})();