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)

length,
    initialData,
    progressiveDone = false,
    contentDispositionFilename = null
  ) {
    this.length = length;
    this.initialData = initialData;
    this.progressiveDone = progressiveDone;
    this.contentDispositionFilename = contentDispositionFilename;
    this._rangeListeners = [];
    this._progressListeners = [];
    this._progressiveReadListeners = [];
    this._progressiveDoneListeners = [];
    this._readyCapability = createPromiseCapability();
  }
  addRangeListener(listener) {
    this._rangeListeners.push(listener);
  }
  addProgressListener(listener) {
    this._progressListeners.push(listener);
  }
  addProgressiveReadListener(listener) {
    this._progressiveReadListeners.push(listener);
  }
  addProgressiveDoneListener(listener) {