Dispatch event when annotations have finished rendering.

This is needed for some smoke tests in mozilla central for testing forms
in pdf.js.

Note: AnnotationLayerBuilder.render() doesn't really need to be async, but
we're talking of making the annotation's render functions async, so this
will make that switch easier.

web/annotation_layer_builder.js

@@ -60,9 +60,11 @@ class AnnotationLayerBuilder {
   /**
    * @param {PageViewport} viewport
    * @param {string} intent (default value is 'display')
+   * @returns {Promise<void>} A promise that is resolved when rendering of the
+   *   annotations is complete.
    */
   render(viewport, intent = "display") {
-    this.pdfPage.getAnnotations({ intent }).then(annotations => {
+    return this.pdfPage.getAnnotations({ intent }).then(annotations => {
       if (this._cancelled) {
         return;
       }