IntersectionObserver: use an idle callback to send notifications.

third_party/WebKit/LayoutTests/http/tests/intersection-observer/iframe-cross-origin.html

Index: third_party/WebKit/LayoutTests/http/tests/intersection-observer/iframe-cross-origin.html
diff --git a/third_party/WebKit/LayoutTests/http/tests/intersection-observer/iframe-cross-origin.html b/third_party/WebKit/LayoutTests/http/tests/intersection-observer/iframe-cross-origin.html
index 3324515a453456a0c2648cce43c58f7226ac0b4f..842d2a428dec31813e421ef5844fdf15516be91c 100644
--- a/third_party/WebKit/LayoutTests/http/tests/intersection-observer/iframe-cross-origin.html
+++ b/third_party/WebKit/LayoutTests/http/tests/intersection-observer/iframe-cross-origin.html
@@ -53,7 +53,8 @@ function handleMessage(event) {
   } else if (event.data.hasOwnProperty('actual')) {
     checkData(event.data.actual, event.data.expected);
   } else if (event.data.hasOwnProperty('DONE')) {
-    finishJSTest();
+    // See LayoutTests/intersection-observer/README for explanation of this requestIdleCallback.
+    requestIdleCallback(finishJSTest, {timeout: 100});
     document.scrollingElement.scrollTop = 0;
   } else {
     requestAnimationFrame(function () { iframe.contentWindow.postMessage("", "*") });
@@ -61,7 +62,11 @@ function handleMessage(event) {
 }
 
 window.addEventListener("message", handleMessage);
+
 iframe.onload = function() {
-  iframe.contentWindow.postMessage("", "*")
-};
+  // See LayoutTests/intersection-observer/README for explanation of double RAF.
+  requestAnimationFrame(() => {
+      requestAnimationFrame(() => { iframe.contentWindow.postMessage("", "*") })
+  });
+}
 </script>