<!DOCTYPE html>
<style>
.fieldset::-webkit-scrollbar {
  width: 30px;
  background: lime;
}
.fieldset::-webkit-scrollbar-thumb {
  background: red;
  border-radius: 12px;
}

.fieldset {
  height: 300px;
  width: 500px;
  overflow: scroll;
  margin: 0;
  padding: 0;
  border: 2px solid gray;
}
</style>
<div class="fieldset">
<div style="height: 600px; width: 400px;"></div>
</div>
<script>
testRunner.waitUntilDone();
const fieldset = document.querySelector('.fieldset');
const rect = fieldset.getBoundingClientRect();
const clickX = rect.right - 20;
const clickY = rect.top + 20;
chrome.gpuBenchmarking.pointerActionSequence([{
  source: 'mouse',
  actions: [
    {name:'pointerDown', x:clickX, y:clickY},
    {name:'pointerMove', x:clickX, y:clickY + 100},
    {name:'pointerUp'}
  ]}], () => { testRunner.notifyDone(); });
</script>