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)

* @property {function} [onPassword] - Callback to request a password if a wrong
 *   or no password was provided. The callback receives two parameters: a
 *   function that should be called with the new password, and a reason (see
 *   {@link PasswordResponses}).
 * @property {function} [onProgress] - 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.
 * @property {function} [onUnsupportedFeature] - Callback for when an
 *   unsupported feature is used in the PDF document. The callback receives an
 *   {@link UNSUPPORTED_FEATURES} argument.
 * @property {Promise<PDFDocumentProxy>} promise - Promise for document loading
 *   task completion.
 * @property {function} destroy - Abort all network requests and destroy
 *   the worker. Returns a promise that is resolved when destruction is
 *   completed.
 */
/**
 * @type {any}
 * @ignore
 */
const PDFDocumentLoadingTask = (function PDFDocumentLoadingTaskClosure() {
  let nextDocumentId = 0;
  /**
   * The loading task controls the operations required to load a PDF document
   * (such as network requests) and provides a way to listen for completion,
   * after which individual pages can be rendered.
   */
  
  class PDFDocumentLoadingTask {