var count = 0;
var max = 10;

var scrollToBottom = function() {
  window.scrollTo(0,document.querySelector(".ytd-page-manager").scrollHeight);
}

var scrollToTop = function() {
 window.scrollTo(0,document.querySelector(".ytd-page-manager"));
}

var scroll = function() {
  scrollToBottom();

  if ( ++count < max)
    setTimeout( function() {scroll()}, 500);
  else
    scrollToTop();
}

scroll();