IntersectionObserver: use an idle callback to send notifications.

third_party/WebKit/LayoutTests/intersection-observer/README

Index: third_party/WebKit/LayoutTests/intersection-observer/README
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+All of the IntersectionObserver tests feature the following idiom:
+
+<script>
+var observer = new IntersectionObserver(...)
+function test_function() {
+  var entries = observer.takeRecords();
+  // Verify entries
+}
+onload = function() {
+  observer.observe(target);
+  requestAnimationFrame(() => { requestAnimationFrame(test_function) });
+}
+
+
+Subsequent steps in the test use a single RAF to give the observer a chance to
+generate notifications, but the double RAF is required in the onload handler.
+Here's the chain of events:
+
+- onload
+  - observer.observe()
+  - First RAF handler is registered
+- BeginFrame
+  - RAF handlers run
+    - Second RAF handler is registered 
+  - UpdateAllLifecyclePhases
+    - IntersectionObserver generates notifications
+- BeginFrame
+  - RAF handlers run
+    - Second RAF handler verifies observer notifications.