Enable a11y audits and fix issues for downloads page.

chrome/browser/ui/webui/downloads_ui_browsertest.js

Index: chrome/browser/ui/webui/downloads_ui_browsertest.js
diff --git a/chrome/browser/ui/webui/downloads_ui_browsertest.js b/chrome/browser/ui/webui/downloads_ui_browsertest.js
index f8de341e9583d6f32632c212acf461028e96f1a7..4eb5bc1dcbb31e88a295546e35ab19b58bfbda5b 100644
--- a/chrome/browser/ui/webui/downloads_ui_browsertest.js
+++ b/chrome/browser/ui/webui/downloads_ui_browsertest.js
@@ -36,6 +36,12 @@ BaseDownloadsWebUITest.prototype = {
     GEN('  SetDeleteAllowed(true);');
   },
 
+  /** @override */
+  runAccessibilityChecks: true,
+
+  /** @override */
+  accessibilityIssuesAreErrors: true,
+
   /**
    * Sends TOTAL_RESULT_COUNT fake downloads to the page. This can't be called
    * in the preLoad, because it requires the global Download object to have