gecko-dev/dom/base/test/viewport_helpers.js

function scaleRatio(scale) {
  return {
    "set": [
      [ "layout.css.devPixelsPerPx", "" + scale ],
      [ "apz.allow_zooming", true ],
      [ "dom.meta-viewport.enabled", true ],
    ]
  };
}

function getViewportInfo(aDisplayWidth, aDisplayHeight) {
  let defaultZoom = {}, allowZoom = {}, minZoom = {}, maxZoom = {},
      width = {}, height = {}, autoSize = {};

  let cwu = SpecialPowers.getDOMWindowUtils(window);
  cwu.getViewportInfo(aDisplayWidth, aDisplayHeight, defaultZoom, allowZoom,
                      minZoom, maxZoom, width, height, autoSize);
  return {
    defaultZoom: defaultZoom.value,
    minZoom: minZoom.value,
    maxZoom: maxZoom.value,
    width: width.value,
    height: height.value,
    autoSize: autoSize.value,
    allowZoom: allowZoom.value
  };
}