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

chrome/browser/resources/md_downloads/manager.html

 <link rel="import" href="chrome://resources/html/cr.html">
 <link rel="import" href="chrome://resources/html/cr/ui.html">
 <link rel="import" href="chrome://resources/html/cr/ui/command.html">
 <link rel="import" href="chrome://resources/html/util.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/polymer/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-list/iron-list.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-header-panel/paper-header-panel.html">
 <link rel="import" href="chrome://downloads/action_service.html">
 <link rel="import" href="chrome://downloads/constants.html">
 <link rel="import" href="chrome://downloads/i18n_setup.html">
 <link rel="import" href="chrome://downloads/item.html">
 <link rel="import" href="chrome://downloads/toolbar.html">
 
 <dom-module id="downloads-manager">
   <template>
     <paper-header-panel id="panel" class="loading">
       <downloads-toolbar class="paper-header" id="toolbar"></downloads-toolbar>
       <iron-list id="downloads-list" items="{{items_}}"
           hidden="[[!hasDownloads_]]">
         <template>
           <downloads-item data="[[item]]" hide-date="[[item.hideDate]]">
           </downloads-item>
         </template>
       </iron-list>
       <div id="no-downloads" hidden="[[hasDownloads_]]">
         <div>
           <div class="illustration"></div>
-          <span><!-- Text populated in Manager#updateAll_(). --></span>
+          <span><!-- Text populated dynamically. --></span>
         </div>
       </div>
     </paper-header-panel>
   </template>
   <link rel="import" type="css" href="chrome://downloads/shared_style.css">
   <link rel="import" type="css" href="chrome://downloads/manager.css">
   <script src="chrome://downloads/manager.js"></script>
 </dom-module>