MD Downloads: add "waterfall" shadow (shows when the list is scrolled)

chrome/browser/resources/md_downloads/manager.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', function() {
   var Manager = Polymer({
     is: 'downloads-manager',
 
     properties: {
       hasDownloads_: {
         observer: 'hasDownloadsChanged_',
         type: Boolean,
       },
 
+      hasShadow_: {
+        type: Boolean,
+        value: false,
+        reflectToAttribute: true,
+      },
+
       items_: {
         type: Array,
         value: function() { return []; },
       },
 
       spinnerActive_: {
         type: Boolean,
         notify: true,
       },
     },
@@ skipping 79 matching lines @@
         this.$.toolbar.onFindCommand();
     },
 
     /** @private */
     onListScroll_: function() {
       var list = this.$['downloads-list'];
       if (list.scrollHeight - list.scrollTop - list.offsetHeight <= 100) {
         // Approaching the end of the scrollback. Attempt to load more items.
         downloads.ActionService.getInstance().loadMore();
       }
+      this.hasShadow_ = list.scrollTop > 0;
     },
 
     /** @private */
     onLoad_: function() {
       cr.ui.decorate('command', cr.ui.Command);
       document.addEventListener('canExecute', this.onCanExecute_.bind(this));
       document.addEventListener('command', this.onCommand_.bind(this));
 
       downloads.ActionService.getInstance().loadMore();
     },
@@ skipping 57 matching lines @@
   Manager.removeItem = function(index) {
     Manager.get().removeItem_(index);
   };
 
   Manager.updateItem = function(index, data) {
     Manager.get().updateItem_(index, data);
   };
 
   return {Manager: Manager};
 });