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)

if (!this.pdfDocument) {
      return Promise.resolve(null);
    }
    if (!this._optionalContentConfigPromise) {
      
      
      return this.pdfDocument.getOptionalContentConfig();
    }
    return this._optionalContentConfigPromise;
  }
  /**
   * @param {Promise<OptionalContentConfig>} promise - A promise that is
   *   resolved with an {@link OptionalContentConfig} instance.
   */
  set optionalContentConfigPromise(promise) {