function getItemHeight(item) {
    var height = item.style.height;
    // Use the fixed animation target height if set, in case the element is
    // currently being animated and we'd get an intermediate height below.
    if (height && height.substr(-2) == 'px') {
      return parseInt(height.substr(0, height.length - 2));
    }
    return item.getBoundingClientRect().height;
  }
Returns the item's height, like offsetHeight but such that it works better
when the page is zoomed. See the similar calculation in @{code cr.ui.List}.
This version also accounts for the animation done in this file.
@param {!Element} item The item to get the height of.
@return {number} The height of the item, calculated with zooming in mind.