MD Downloads: track downloads in C++, dispatch discrete JS updates

chrome/test/data/webui/md_downloads/layout_tests.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.
 
 cr.define('downloads.layout_tests', function() {
   function registerTests() {
     suite('LayoutTests', function() {
       /** @type {!downloads.Manager} */
       var manager;
 
       setup(function() {
         PolymerTest.clearBody();
         manager = document.createElement('downloads-manager');
         document.body.appendChild(manager);
+        assertEquals(manager, downloads.Manager.get());
       });
 
       test('long URLs ellide', function() {
-        manager.updateAll_([{
+        downloads.Manager.insertItems(0, [{
           file_name: 'file name',
           state: downloads.States.COMPLETE,
-          url: 'a'.repeat(10000),
+          url: 'a'.repeat(1000),
         }]);
 
         Polymer.dom.flush();
 
         var item = manager.$$('downloads-item');
         assertLT(item.$$('#url').offsetWidth, item.offsetWidth);
       });
     });
   }
 
   return {
     registerTests: registerTests,
   };
 });