code: function processDimensionsResults(controller, boxes) {
    if (boxes && boxes.length > 0) {
      screenshot.create(controller, boxes);
    }
  }
docs: Callback function for parsing the results of testing for cropped elements.
  
  This function calls the screenshot.create method if there is at least one
  box.
  
  @param {array of array of int} boxes
  @param {MozmillController} controller