MD Downloads: use <iron-list> to render download items

chrome/browser/resources/md_downloads/item.css

 /* 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. */
 
 :host {
   display: flex;
   flex-direction: column;
-  margin: 12px 0;
 }
 
 #date {
   color: #7c7c7c;
   font-size: 100%;
   font-weight: 500;
-  margin: 18px auto 16px;  /* This top margin + :host top margin = 30px. */
+  margin: 24px auto 10px;
   width: var(--downloads-item-width);
 }
 
 #content {
   background: white;
   border-radius: 2px;
   display: flex;
   flex: none;
-  margin: 0 auto;
+  margin: 6px auto;
   min-height: 103px;
   width: var(--downloads-item-width);
 }
 
 #content.is-active {
   box-shadow: 0 2px 2px 0 rgba(0, 0, 0, .05), 0 1px 4px 0 rgba(0, 0, 0, .08),
               0 3px 1px -2px rgba(0, 0, 0, .2);
 }
 
 #details {
@@ skipping 152 matching lines @@
   width: 16px;
 }
 
 #incognito {
   bottom: 20px;
   content: -webkit-image-set(url(1x/incognito_marker.png) 1x,
                              url(2x/incognito_marker.png) 2x);
   position: absolute;
   right: 10px;
 }