Bug 1684708 - Migrate helper_scrollby_bug1531796.html to async/await style. r=botond

Depends on D100617

Differential Revision: https://phabricator.services.mozilla.com/D100618

gfx/layers/apz/test/mochitest/helper_scrollby_bug1531796.html

@@ -9,18 +9,19 @@
   <script src="/tests/SimpleTest/paint_listener.js"></script>
   <script type="application/javascript">
 
-function* test(testDriver) {
+async function test() {
   const maxSteps = 20;
   let scrollPerStep = 40;
   for (let step = 0; step < maxSteps; step++) {
     window.scrollBy(0, scrollPerStep);
-    window.requestAnimationFrame(testDriver);
-    yield;
+    await waitForFrame();
   }
   is(window.scrollY, maxSteps * scrollPerStep, "Scrolled by the expected amount");
 }
 
-waitUntilApzStable().then(runContinuation(test)).then(subtestDone, subtestFailed);
+waitUntilApzStable()
+.then(test)
+.then(subtestDone, subtestFailed);
 
   </script>
   <style>