Modernize PDFMetadata class

 - Convert `PDFMetadata` to an ES2015 class

 - Convert the ad-hoc type documentation to tool-readable JSDoc

 - Add a little more detail in doc comments about what
   `documentFingerprint` is

src/annotator/plugin/pdf-metadata.js

@@ -3,15 +3,33 @@
 const { normalizeURI } = require('../util/url');
 
 /**
- * This PDFMetadata service extracts metadata about a loading/loaded PDF
- * document from a PDF.js PDFViewerApplication object.
+ * @typedef Link
+ * @prop {string} href
+ */
+
+/**
+ * @typedef Metadata
+ * @prop {string} title - The document title
+ * @prop {Link[]} link - Array of URIs associated with this document
+ * @prop {string} documentFingerprint - The fingerprint of this PDF. This is
+ *   referred to as the "File Identifier" in the PDF spec. It may be a hash of
+ *   part of the content if the PDF file does not have a File Identifier.
+ */
+
+/**
+ * PDFMetadata extracts metadata about a loading/loaded PDF document from a
+ * PDF.js PDFViewerApplication object.
+ */
+class PDFMetadata {
+  /**
+   * Construct a `PDFMetadata` that returns URIs/metadata associated with a
+   * given PDF viewer.
    *
- * This hides from users of this service the need to wait until the PDF document
- * is loaded before extracting the relevant metadata.
+   * @param {PDFViewerApplication} app
    */
-function PDFMetadata(app) {
-  this._loaded = new Promise(function (resolve) {
-    var finish = function () {
+  constructor(app) {
+    this._loaded = new Promise(resolve => {
+      var finish = () => {
         window.removeEventListener('documentload', finish);
         resolve(app);
       };
@@ -22,30 +40,36 @@ function PDFMetadata(app) {
         window.addEventListener('documentload', finish);
       }
     });
-}
+  }
 
-/**
- * Returns a promise of the URI of the loaded PDF.
+  /**
+   * Return the URI of the PDF.
+   *
+   * If the PDF is currently loading, the returned promise resolves once loading
+   * is complete.
+   *
+   * @return {Promise<string>}
    */
-PDFMetadata.prototype.getUri = function () {
-  return this._loaded.then(function (app) {
+  getUri() {
+    return this._loaded.then(app => {
       var uri = getPDFURL(app);
       if (!uri) {
         uri = fingerprintToURN(app.documentFingerprint);
       }
       return uri;
     });
-};
+  }
 
-/**
- * Returns a promise of a metadata object, containing:
+  /**
+   * Returns metadata about the document.
+   *
+   * If the PDF is currently loading, the returned promise resolves once loading
+   * is complete.
    *
- * title(string) - The document title
- * link(array) - An array of link objects representing URIs for the document
- * documentFingerprint(string) - The document fingerprint
+   * @return {Promise<Metadata>}
    */
-PDFMetadata.prototype.getMetadata = function () {
-  return this._loaded.then(function (app) {
+  getMetadata() {
+    return this._loaded.then(app => {
       var title = document.title;
 
       if (app.metadata && app.metadata.has('dc:title') && app.metadata.get('dc:title') !== 'Untitled') {
@@ -69,7 +93,8 @@ PDFMetadata.prototype.getMetadata = function () {
         documentFingerprint: app.documentFingerprint,
       };
     });
-};
+  }
+}
 
 function fingerprintToURN(fingerprint) {
   return 'urn:x-pdf:' + String(fingerprint);