TableRow = function(trElement, rowIndex) {
  this.index = rowIndex;
  this.element = trElement;
  this.columns = [];
}
Class representing a logical table row: a tr element and any cells
that appear in that row.
@param {!Element} trElement This rows's underlying TR element.
@param {number} rowIndex This row's index in its parent table.
@constructor
@final