static inheritProperty_(parents, cb) {
    for (let i = parents.length - 1; i >= 0; i--) {
      const val = cb(parents[i]);
      if (val || val === 0) {
        return val;
      }
    }
    return null;
  }
Inherits a property from the parent Cue elements.  If the value is falsy,
it is assumed to be inherited from the parent. This returns null if the
value isn't found.

@param {!Array<!shaka.text.Cue>} parents
@param {function(!shaka.text.Cue):?T} cb
@return {?T}
@template T
@private