chaptersEqual = (a, b) => {
      return (!a && !b) || (a.id === b.id && a.title === b.title &&
          a.startTime === b.startTime && a.endTime === b.endTime);
    }
Does a value compare on chapters.
@param {shaka.extern.Chapter} a
@param {shaka.extern.Chapter} b
@return {boolean}