Allow the PDF page indicator to accept a list of page labels

elements/viewer-page-indicator/viewer-page-indicator.js

 // Copyright 2014 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.
 
 Polymer('viewer-page-indicator', {
-  text: '1',
+  label: '1',
+  index: 0,
   timerId: undefined,
+  pageLabels: null,
   ready: function() {
     var callback = this.fadeIn.bind(this, 2000);
     window.addEventListener('scroll', function() {
       requestAnimationFrame(callback);
     });
   },
   initialFadeIn: function() {
     this.fadeIn(6000);
   },
   fadeIn: function(displayTime) {
     var percent = window.scrollY /
         (document.body.scrollHeight -
          document.documentElement.clientHeight);
     this.style.top = percent *
         (document.documentElement.clientHeight - this.offsetHeight) + 'px';
     this.style.opacity = 1;
     clearTimeout(this.timerId);
 
     this.timerId = setTimeout(function() {
       this.style.opacity = 0;
       this.timerId = undefined;
     }.bind(this), displayTime);
+  },
+  pageLabelsChanged: function() {
+    this.indexChanged();
+  },
+  indexChanged: function() {
+    if (this.pageLabels)
+      this.label = this.pageLabels[this.index];
+    else
+      this.label = String(this.index + 1);
   }
 });