Fix IDL definition of LoadDocumentCallback

Dictionary arguments cannot be nullable in Web IDL:

> "Although dictionary types can in general be nullable, they cannot when
used as the type of an operation argument or a dictionary member."
> https://webidl.spec.whatwg.org/#dfn-nullable-type

Note: this was missed before because the webidl2 checker did not validate
arguments in callback definitions. It now does.

Author: tidoust

index.html

@@ -6572,7 +6572,7 @@ <h3>LoadDocumentCallback</h3>
       <pre class="idl">
         callback LoadDocumentCallback = Promise&lt;RemoteDocument> (
           USVString url,
-          optional LoadDocumentOptions? options
+          optional LoadDocumentOptions options = {}
         );
       </pre>