Enable a11y audits and fix issues for downloads page.

BUG=265583

Review URL: https://codereview.chromium.org/598533004

Cr-Commit-Position: refs/heads/master@{#296513}

chrome/browser/resources/downloads/downloads.js

@@ -323,6 +323,7 @@ function Download(download) {
   }
 
   this.nodeImg_ = createElementWithClassName('img', 'icon');
+  this.nodeImg_.alt = '';
   this.safe_.appendChild(this.nodeImg_);
 
   // FileLink is used for completed downloads, otherwise we show FileName.
@@ -405,6 +406,7 @@ function Download(download) {
   this.node.appendChild(this.danger_);
 
   this.dangerNodeImg_ = createElementWithClassName('img', 'icon');
+  this.dangerNodeImg_.alt = '';
   this.danger_.appendChild(this.dangerNodeImg_);
 
   this.dangerDesc_ = document.createElement('div');

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