Made nextElementSibling and previousElementSibling nullable

third_party/WebKit/Source/core/dom/NonDocumentTypeChildNode.idl

Index: third_party/WebKit/Source/core/dom/NonDocumentTypeChildNode.idl
diff --git a/third_party/WebKit/Source/core/dom/NonDocumentTypeChildNode.idl b/third_party/WebKit/Source/core/dom/NonDocumentTypeChildNode.idl
index b2883c41321802b34476e50ab63670f90fa6ad7a..ff4cacc4b3ad2a773eeab6ee3046e402a83e074e 100644
--- a/third_party/WebKit/Source/core/dom/NonDocumentTypeChildNode.idl
+++ b/third_party/WebKit/Source/core/dom/NonDocumentTypeChildNode.idl
@@ -8,6 +8,6 @@
     LegacyTreatAsPartialInterface,
     NoInterfaceObject, // Always used on target of 'implements'
 ] interface NonDocumentTypeChildNode {
-    [PerWorldBindings] readonly attribute Element previousElementSibling;
-    [PerWorldBindings] readonly attribute Element nextElementSibling;
+    [PerWorldBindings] readonly attribute Element? previousElementSibling;
+    [PerWorldBindings] readonly attribute Element? nextElementSibling;
 };