Convert IntersectionObserver tests to use testRunner.runIdleTasks.

third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js

 // Some of the js-test.js boilerplate will add stuff to the top of the document early
 // enough to screw with frame offsets that are measured by the test.  Delay all that
 // jazz until the actual test code is finished.
 setPrintTestResultsLazily();
 self.jsTestIsAsync = true;
 
 function rectArea(rect) {
   return (rect.left - rect.right) * (rect.bottom - rect.top);
 }
 
+// waitForNotification is a requestIdleCallback wrapped in a setTimeout wrapped in a
+// requestAnimationFrame.  What in the wide, wide world of sports is going on here?
+//
+// Here's the order of events:
+//
+// - firstTestFunction
+//   - Change layout to generate new IntersectionObserver notifications.
+//   - waitForNotification(secondTestFunction)
+//     - requestAnimationFrame
+// - BeginFrame
+//   - requestAnimationFrame handler runs.
+//     - setTimeout
+//   - FrameView::updateAllLifecyclePhases
+//     - IntersectionObserver generates notification based on the new layout.
+//       - Post idle task to deliver notification.
+//   - setTimeout handler runs.
+//     - testRunner.runIdleTasks or requestIdleCallback.
+//   - Idle tasks run -- more or less immediately if (self.testRunner),
+//     possibly delayed if (!self.testRunner).
+//     - IntersectionObserver notifications are delivered.
+//     - secondTestFunction
+//       - Verify notifications generated by firstTestFunction.
+//       - Change layout to generate new IntersectionObserver notifications.
+//       - waitForNotification(thirdTestFunction)
+//
+// Note that this should work equally well in these operation conditions:
+//
+//   - layout test using single-threaded compositing (the default)
+//   - layout test using multi-threaded compositing (virtual/threaded/)
+//   - Not in a layout test and using multi-threaded compositing (the only configuration we ship)
+function waitForNotification(f) {
+  requestAnimationFrame(() => {
+    setTimeout(() => {
+      if (self.testRunner)
+        testRunner.runIdleTasks(f);
+      else
+        requestIdleCallback(f);
+    })
+  })
+}
+
 function rectToString(rect) {
   return "[" + rect.left + ", " + rect.right + ", " + rect.top + ", " + rect.bottom + "]";
 }
 
 function entryToString(entry) {
   var ratio = ((entry.intersectionRect.width * entry.intersectionRect.height) /
                (entry.boundingClientRect.width * entry.boundingClientRect.height));
   return (
       "boundingClientRect=" + rectToString(entry.boundingClientRect) + "\n" +
       "intersectionRect=" + rectToString(entry.intersectionRect) + "\n" +
       "visibleRatio=" + ratio + "\n" +
       "rootBounds=" + rectToString(entry.rootBounds) + "\n" +
       "target=" + entry.target + "\n" +
       "time=" + entry.time);
 }
 
 function intersectionRatio(entry) {
   var targetArea = rectArea(entry.boundingClientRect);
   if (!targetArea)
     return 0;
   return rectArea(entry.intersectionRect) / targetArea;
 }