Set rootBounds to null for cross-origin observations.

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

+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
+  };
+}