code: function createEnterVRButton (onClick) {
    var vrButton;
    var wrapper;
  
    // Create elements.
    wrapper = document.createElement('div');
    wrapper.classList.add(ENTER_VR_CLASS);
    wrapper.setAttribute(constants.AFRAME_INJECTED, '');
    vrButton = document.createElement('button');
    vrButton.className = ENTER_VR_BTN_CLASS;
    vrButton.setAttribute('title',
      'Enter VR mode with a headset or fullscreen mode on a desktop. ' +
      'Visit https://webvr.rocks or https://webvr.info for more information.');
    vrButton.setAttribute(constants.AFRAME_INJECTED, '');
  
    // Insert elements.
    wrapper.appendChild(vrButton);
    vrButton.addEventListener('click', function (evt) {
      onClick();
      evt.stopPropagation();
    });
    return wrapper;
  }
docs: Create a button that when clicked will enter into stereo-rendering mode for VR.
  
  Structure: <div><button></div>
  
  @param {function} onClick - click event handler
  @returns {Element} Wrapper <div>.