You have to break down the big problem into smaller problems.

First, you need to get your video's duration :

var vid = document.getElementById("myVideo");
duration = vid.duration;

Next, you need to get the scroll amount in percentage

function getVerticalScrollPercentage (elm) {
  var p = elm.parentNode
  return (elm.scrollTop || p.scrollTop) / (p.scrollHeight - p.clientHeight ) * 100
}

Next, you need to dynamically set your video's currentTime (in seconds), you can do it this way :

vid.currentTime = duration * percentage / 100;

And finally, you need to set the currentTime again, whenever the scroll amount changes. That is achieved by using an event listener, on the body for example.

object.addEventListener("scroll", myScript);

Now put it all together :)