MD Downloads: add first meaningful paint metrics to UMA

chrome/browser/resources/md_downloads/downloads.js

 // Copyright 2015 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.
 
 window.addEventListener('load', function() {
-  downloads.Manager.onLoad();
-  document.fonts.load('bold 12px Roboto');
+  downloads.Manager.onLoad().then(function() {
+    requestIdleCallback(function() {
+      chrome.send('metricsHandler:recordTime', [
+        'Download.ResultsRenderedTime',
+        window.performance.now()
+      ]);
+      document.fonts.load('bold 12px Roboto');
+    });
+  });
 });