DevTools: Add aXe test runner and accessibility test

After submiting accessbility fixes for the Sensors tool and Performance
Monitor, we should aim to not regress these improvements.

This change adds the AxeCoreTestRunner module which uses aXe to run
basic accessbility checks on the UI.

It also adds a test that runs aXe against the Performance Monitor and
Sensors tool since they have already received an initial accessibility
pass and are not reporting any violations in aXe.

Change-Id: I6443d5014d9200a9295746aaec7a71b4659a6417
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1534551
Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Commit-Queue: Amanda Baker <ambake@microsoft.com>
Cr-Original-Commit-Position: refs/heads/master@{#645823}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f854b6f5a7a1e16fe5880c40e8055e92eca37943
diff --git a/scripts/check_gn.js b/scripts/check_gn.js
index e17ce41..cbf2f87 100644
--- a/scripts/check_gn.js
+++ b/scripts/check_gn.js
@@ -71,7 +71,7 @@
  */
 function checkAllDevToolsFiles() {
   const errors = [];
-  const excludedFiles = ['InspectorBackendCommands.js', 'SupportedCSSProperties.js', 'ARIAProperties.js'];
+  const excludedFiles = ['InspectorBackendCommands.js', 'SupportedCSSProperties.js', 'ARIAProperties.js', 'axe.js'];
   const gnVariable = 'all_devtools_files';
   const lines = selectGNLines(`${gnVariable} = [`, ']').map(path.normalize);
   if (!lines.length) {