getParentTableRow_(elem) {
    var parentTR = elem.parentElement;
    while (parentTR.tagName != goog.dom.TagName.TR) {
      parentTR = parentTR.parentElement;
    }

    return parentTR;
  }
Returns the parent table row of the given element.
@param {HTMLElement} elem The element for which we need to find the parent
    table row.
@return {!Element} The parent table row.
@private