code: function createPromiseCapability() {
    var capability = {};
    capability.promise = new Promise(function (resolve, reject) {
      capability.resolve = resolve;
      capability.reject = reject;
    });
    return capability;
  }
docs: Promise Capability object.
  
  @typedef {Object} PromiseCapability
  @property {Promise} promise - A promise object.
  @property {function} resolve - Fullfills the promise.
  @property {function} reject - Rejects the promise.
   
  Creates a promise capability object.
  @alias PDFJS.createPromiseCapability
  
  @return {PromiseCapability} A capability object contains:
  - a Promise, resolve and reject methods.