function checkRow(row) {
    if (row && (row.rowIndex < topRowIndex || row.rowIndex > bottomRowIndex)) {
      row.setAttribute('aria-hidden', 'true');
      hiddenRows.push(row);
    }
  }
Ensure aria-hidden is set on any selection rows that are offscreen.

The attribute aria-hidden is set to 'true' so that hidden rows are ignored
by screen readers.  We keep a list of currently hidden rows so they can be
reset each time this function is called as the selection and/or scrolling
may have changed.

@param {number} topRowIndex Index of top row on screen.
@param {number} bottomRowIndex Index of bottom row on screen.