getNumberOfClosedCaptionsInChannel(channelId) {
    const channel = this.closedCaptionsMap_.get(channelId);
    return channel ? channel.size : 0;
  }
Get the number of closed caption cues for a given channel. If there is
no channel for the given channel id, this will return 0.

This function is for TESTING ONLY. DO NOT USE in the library.

@param {string} channelId
@return {number}