Material WebUI: Switch to poly-icons to generate iconsets

ui/webui/resources/cr_elements/cr_expand_button/cr_expand_button.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.
 
 /**
  * @fileoverview
  * 'cr-expand-button' is a chrome-specific wrapper around paper-icon-button that
  * toggles between an opened (expanded) and closed state.
  *
  * Example:
@@ skipping 18 matching lines @@
      * If true, the button will be disabled and greyed out.
      */
     disabled: {
       type: Boolean,
       value: false,
       reflectToAttribute: true
     },
   },
 
   iconName_: function(expanded) {
-    return expanded ? 'expand-less' : 'expand-more';
+    return expanded ? 'cr:expand-less' : 'cr:expand-more';
   }
 });