I was wondering if WebGL is considered stable enough to be enabled by default. It makes some PDFs render much faster and it has been off by default for a while now, so we have had quite some time to test it.

If we decide to enable WebGL by default, we should not forget to update https://github.com/mozilla/pdf.js/blob/master/extensions/chromium/preferences_schema.json#L39 as well.

/cc @yurydelendik 
/cc @Snuffleupagus (because IIRC you were testing WebGL too)

* @type {function}
       */
      this.onPassword = null;
      /**
       * Callback to be able to monitor the loading progress of the PDF file
       * (necessary to implement e.g. a loading bar). The callback receives
       * an {Object} with the properties `loaded` ({number}) and `total`
       * ({number}) that indicate how many bytes are loaded.
       * @type {function}
       */
      this.onProgress = null;
      /**
       * Callback for when an unsupported feature is used in the PDF document.
       * The callback receives an {@link UNSUPPORTED_FEATURES} argument.
       * @type {function}
       */
      this.onUnsupportedFeature = null;
    }
    /**
     * Promise for document loading task completion.
     * @type {Promise<PDFDocumentProxy>}
     */
    get promise() {
      return this._capability.promise;
    }
    /**
     * @returns {Promise<void>} A promise that is resolved when destruction is
     *   completed.
     */
    destroy() {