code: function updateSigninStatus(isSignedIn) {
  	if (isSignedIn) {
  		authorizeButton.style.display = 'none';
  		signoutButton.style.display = 'block';
  		listFiles();
  	} else {
  		authorizeButton.style.display = 'block';
  		signoutButton.style.display = 'none';
  	}
  }
docs: Called when the signed in status changes, to update the UI
  appropriately. After a sign-in, the API is called.