function toCssPx(pixels) {
    if (!window.isFinite(pixels))
      console.error('Pixel value is not a number: ' + pixels);
    return Math.round(pixels) + 'px';
  }
Takes a number and spits out a value CSS will be happy with. To avoid
subpixel layout issues, the value is rounded to the nearest integral value.
@param {number} pixels The number of pixels.
@return {string} e.g. '16px'.