Use the assetpath for viewer-button in the PDF viewer toolbar

elements/viewer-button/viewer-button.html

 <polymer-element name="viewer-button" attributes="src latchable">
   <template>
     <link rel="stylesheet" href="viewer-button.css">
     <div id="icon"></div>
   </template>
   <script>
     (function() {
     var dpi = '';
 
     Polymer('viewer-button', {
       src: '',
       latchable: false,
       ready: function() {
         if (!dpi) {
           var mql = window.matchMedia('(-webkit-min-device-pixel-ratio: 1.3');
           dpi = mql.matches ? 'hi' : 'low';
         }
       },
       srcChanged: function() {
         if (this.src) {
           this.$.icon.style.backgroundImage =
-              'url(elements/viewer-button/img/' + dpi + 'DPI/' + this.src + ')';
+              'url(' + this.getAttribute('assetpath') + 'img/' + dpi +
+                  'DPI/' + this.src + ')';
         } else {
           this.$.icon.style.backgroundImage = '';
         }
       },
       latchableChanged: function() {
         if (this.latchable)
           this.classList.add('latchable');
         else
           this.classList.remove('latchable');
       },
     });
     })();
   </script>
 </polymer-element>