Convert IntersectionObserver tests to use testRunner.runIdleTasks.

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

Index: third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js
diff --git a/third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js b/third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js
index 7e2612d49f5806c2244c5d96c295b16c54ef6a02..6fd2361eef0914b9ca25690730aef02da71e29ce 100644
--- a/third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js
+++ b/third_party/WebKit/LayoutTests/intersection-observer/helper-functions.js
@@ -8,6 +8,47 @@ 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 + "]";
 }