getCopy() {
    const clone = (entry) => {
      return {
        timestamp: entry.timestamp,
        state: entry.state,
        duration: entry.duration,
      };
    };

    const copy = [];
    for (const entry of this.closed_) {
      copy.push(clone(entry));
    }
    if (this.open_) {
      copy.push(clone(this.open_));
    }

    return copy;
  }
Get a copy of each state change entry in the history. A copy of each entry
is created to break the reference to the internal data.

@return {!Array<shaka.extern.StateChange>}