function assertAttachSeamlessIframeSizesCorrectly(fieldObj, iframe) {
  const size = style.getSize(fieldObj.getOriginalElement());
  fieldObj.attachIframe(iframe);
  const newSize = style.getSize(fieldObj.getOriginalElement());

  assertEquals(size.width, newSize.width);
  assertEquals(size.height, newSize.height);
}
Make sure that the original field element for the given Field has
the same size before and after attaching the given iframe. If this is not
true, then the field will fidget while we're initializing the field,
and that's not what we want.
@param {?Field} fieldObj The field.
@param {?HTMLIFrameElement} iframe The iframe.
@suppress {visibility} suppression added to enable type checking