Update interfaces/webxr.idl (#22500)

Source: https://github.com/tidoust/reffy-reports/blob/a71fa36/ed/idl/webxr.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/667838166

Author: autofoolip

interfaces/webxr.idl

@@ -258,7 +258,7 @@ interface XRReferenceSpaceEvent : Event {
 
 dictionary XRReferenceSpaceEventInit : EventInit {
   required XRReferenceSpace referenceSpace;
-  XRRigidTransform transform;
+  XRRigidTransform? transform = null;
 };
 
 dictionary XRPermissionDescriptor: PermissionDescriptor {