Bug 1686372 - Migrate helper_drag_root_scrollbar.html to async/await style. r=botond

Depends on D101528

Differential Revision: https://phabricator.services.mozilla.com/D101529

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

@@ -15,12 +15,14 @@
   </style>
   <script type="text/javascript">
 
-function* test(testDriver) {
-  window.addEventListener("scroll", () => setTimeout(testDriver, 0), {once: true});
+async function test() {
+  let scrollPromise = new Promise(resolve => {
+    window.addEventListener("scroll", resolve, {once: true});
+  });
 
   // Do the scroll in one increment so that when the scroll event fires
   // we're done all the scrolling we're going to do.
-  var dragFinisher = yield* dragVerticalScrollbar(window, testDriver, 20, 20);
+  var dragFinisher = await promiseVerticalScrollbarDrag(window, 20, 20);
   if (!dragFinisher) {
     ok(true, "No scrollbar, can't do this test");
     return;
@@ -29,12 +31,12 @@ function* test(testDriver) {
   // the events above might be stuck in APZ input queue for a bit until the
   // layer is activated, so we wait here until the scroll event listener is
   // triggered.
-  yield;
+  await scrollPromise;
 
-  yield* dragFinisher();
+  await dragFinisher();
 
   // Flush everything just to be safe
-  yield flushApzRepaints(testDriver);
+  await promiseApzRepaintsFlushed();
 
   // After dragging the scrollbar 20px on a 1000px-high viewport, we should
   // have scrolled approx 2% of the 5000px high content. There might have been
@@ -48,7 +50,7 @@ function* test(testDriver) {
 }
 
 waitUntilApzStable()
-.then(runContinuation(test))
+.then(test)
 .then(subtestDone, subtestFailed);
 
   </script>