Sync the MutationObserver and MutationRecord interfaces with the spec

Source/core/dom/MutationRecord.idl

Index: Source/core/dom/MutationRecord.idl
diff --git a/Source/core/dom/MutationRecord.idl b/Source/core/dom/MutationRecord.idl
index 1be9ca99258fc472072856333fec8b5432e99432..8880c84824ef6d827ad1c0b8e65f3416fcd251f5 100644
--- a/Source/core/dom/MutationRecord.idl
+++ b/Source/core/dom/MutationRecord.idl
@@ -28,19 +28,18 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+// https://dom.spec.whatwg.org/#interface-mutationrecord
+
 [
     WillBeGarbageCollected,
 ] interface MutationRecord {
     readonly attribute DOMString type;
     readonly attribute Node target;
-
     readonly attribute NodeList addedNodes;
     readonly attribute NodeList removedNodes;
-    readonly attribute Node previousSibling;
-    readonly attribute Node nextSibling;
-
+    readonly attribute Node? previousSibling;
+    readonly attribute Node? nextSibling;
     readonly attribute DOMString? attributeName;
     readonly attribute DOMString? attributeNamespace;
-
     readonly attribute DOMString? oldValue;
 };