Convert IntersectionObserver tests to use testRunner.runIdleTasks.

third_party/WebKit/LayoutTests/intersection-observer/root-margin.html

Index: third_party/WebKit/LayoutTests/intersection-observer/root-margin.html
diff --git a/third_party/WebKit/LayoutTests/intersection-observer/root-margin.html b/third_party/WebKit/LayoutTests/intersection-observer/root-margin.html
index dda62ea367ecfadba79eaaab46948248591c025b..d72dbad8ac819d8a9754ce9fc492e47ae706f937 100644
--- a/third_party/WebKit/LayoutTests/intersection-observer/root-margin.html
+++ b/third_party/WebKit/LayoutTests/intersection-observer/root-margin.html
@@ -27,19 +27,16 @@ onload = function() {
   observer.observe(target);
   entries = entries.concat(observer.takeRecords());
   shouldBeEqualToNumber("entries.length", 0);
-  // See README for explanation of double RAF.
-  requestAnimationFrame(() => { requestAnimationFrame(step0) });
+  waitForNotification(step0);
 }
 
 function step0() {
-  entries = entries.concat(observer.takeRecords());
   shouldBeEqualToNumber("entries.length", 0);
   document.scrollingElement.scrollLeft = 100;
-  requestAnimationFrame(step1);
+  waitForNotification(step1);
 }
 
 function step1() {
-  entries = entries.concat(observer.takeRecords());
   shouldBeEqualToNumber("entries.length", 1);
   if (entries.length > 0) {
     shouldBeEqualToNumber("entries.length", 1);
@@ -58,18 +55,16 @@ function step1() {
     shouldEvaluateToSameObject("entries[0].target", target);
   }
   document.scrollingElement.scrollTop = 800;
-  requestAnimationFrame(step2);
+  waitForNotification(step2);
 }
 
 function step2() {
-  entries = entries.concat(observer.takeRecords());
   shouldBeEqualToNumber("entries.length", 1);
   document.scrollingElement.scrollTop = 900;
-  requestAnimationFrame(step3);
+  waitForNotification(step3);
 }
 
 function step3() {
-  entries = entries.concat(observer.takeRecords());
   shouldBeEqualToNumber("entries.length", 2);
   if (entries.length > 1) {
     shouldBeEqualToNumber("entries[1].boundingClientRect.left", 912);