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)

this._pages = [];
    this._currentPageNumber = 1;
    this._currentScale = UNKNOWN_SCALE;
    this._currentScaleValue = null;
    this._pageLabels = null;
    this._buffer = new PDFPageViewBuffer(DEFAULT_CACHE_SIZE);
    this._location = null;
    this._pagesRotation = 0;
    this._optionalContentConfigPromise = null;
    this._pagesRequests = new WeakMap();
    this._firstPageCapability = createPromiseCapability();
    this._onePageRenderedCapability = createPromiseCapability();
    this._pagesCapability = createPromiseCapability();
    this._scrollMode = ScrollMode.VERTICAL;
    this._spreadMode = SpreadMode.NONE;
    if (this._onBeforeDraw) {
      this.eventBus._off("pagerender", this._onBeforeDraw);
      this._onBeforeDraw = null;
    }
    if (this._onAfterDraw) {
      this.eventBus._off("pagerendered", this._onAfterDraw);
      this._onAfterDraw = null;
    }
    
    this.viewer.textContent = "";
    
    this._updateScrollMode();
  }
  _scrollUpdate() {