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)

let scale = parseFloat(value);
    if (scale > 0) {
      this._setScaleUpdatePages(scale, value, noScroll,  false);
    } else {
      const currentPage = this._pages[this._currentPageNumber - 1];
      if (!currentPage) {
        return;
      }
      const noPadding = this.isInPresentationMode || this.removePageBorders;
      let hPadding = noPadding ? 0 : SCROLLBAR_PADDING;
      let vPadding = noPadding ? 0 : VERTICAL_PADDING;
      if (!noPadding && this._isScrollModeHorizontal) {
        [hPadding, vPadding] = [vPadding, hPadding]; 
      }
      const pageWidthScale =
        (((this.container.clientWidth - hPadding) / currentPage.width) *
          currentPage.scale) /
        this._pageWidthScaleFactor;
      const pageHeightScale =
        ((this.container.clientHeight - vPadding) / currentPage.height) *
        currentPage.scale;
      switch (value) {
        case "page-actual":
          scale = 1;
          break;
        case "page-width":