Set rootBounds to null for cross-origin observations.

third_party/WebKit/LayoutTests/http/tests/intersection-observer/resources/helper-functions.js

+// Some of the js-test.js boilerplate will add stuff to the top of the document early
+// enough to screw with frame offsets that are measured by the test.  Delay all that
+// jazz until the actual test code is finished.
+if (window.setPrintTestResultsLazily)
+  setPrintTestResultsLazily();
+var delayDescription = description || null;
+var descriptionString = "";
+var delayIsSuccessfullyParsed = isSuccessfullyParsed || null;
+var isSuccessfullyParsed = function() {}
+var description = function(msg) { descriptionString = msg }

[ojan, 2016/01/21 23:56:43]

I don't think you need to do all this monkey patching of js-test. Instead, I
think you just need:
window.jsTestIsAsync = true;

Then when your test is done, call finishJSTest().

[szager1, 2016/01/22 00:23:39]

I tried that; didn't work :)

+
+if (window.testRunner)
+  testRunner.waitUntilDone();
+
+function clientRectToJson(rect) {
+  if (!rect)
+    return null;
+  return {
+    top: rect.top,
+    right: rect.right,
+    bottom: rect.bottom,
+    left: rect.left,
+    width: rect.width,
+    height: rect.height
+  };
+}
+
+function coordinatesToClientRectJson(top, right, bottom, left) {
+  return {
+    top: top,
+    right: right,
+    bottom: bottom,
+    left: left,
+    width: right - left,
+    height: bottom - top
+  };
+}
+
+function entryToJson(entry) {
+  return {
+    boundingClientRect: clientRectToJson(entry.boundingClientRect),
+    intersectionRect: clientRectToJson(entry.intersectionRect),
+    rootBounds: clientRectToJson(entry.rootBounds),
+    time: entry.time,
+    target: entry.target.id
+  };
+}
+
+function finishTest() {
+  if (descriptionString)
+    delayDescription(descriptionString);
+  delayIsSuccessfullyParsed();
+  finishJSTest();
+  if (window.testRunner)
+    testRunner.notifyDone();
+}