controlEdm_(pts) {
    const buf = this.displayedMemory_;
    let parsedClosedCaption = null;
    if (this.type_ !== shaka.cea.Cea608DataChannel.CaptionType.TEXT) {
      // Clearing displayed memory means we now know how long
      // its contents were displayed, so force it out.
      parsedClosedCaption = buf.forceEmit(this.prevEndTime_, pts);
    }
    buf.resetAllRows();
    return parsedClosedCaption;
  }
Handles EDM - Erase Displayed Mem
Mode check:
EDM affects all captioning modes (but not Text mode);
@param {number} pts
@return {?shaka.extern.ICaptionDecoder.ClosedCaption}
@private