Revert of IntersectionObserver: use an idle callback to send notifications.

third_party/WebKit/LayoutTests/intersection-observer/README

-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.