makeUnsupportedButton_(buttonName, unsupportedReason) {
    const button = this.addButton(buttonName, () => {});
    button.setAttribute('disabled', '');

    // Tooltips don't work on disabled buttons (on some platforms), so
    // the button itself has to be "uprooted" and placed in a synthetic div
    // specifically to attach the tooltip to.
    const attachPoint = document.createElement('div');
    if (button.parentElement) {
      button.parentElement.removeChild(button);
    }
    attachPoint.classList.add('tooltip-attach-point');
    attachPoint.appendChild(button);
    this.actions_.appendChild(attachPoint);
    shakaDemo.Tooltips.make(attachPoint, unsupportedReason);

    return button;
  }
Make a button that represents the lack of a working button.
@param {?string} buttonName
@param {string} unsupportedReason
@return {!Element}
@private