Sync HTML element interfaces O-Q with the spec

Source/core/html/HTMLOutputElement.idl

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+// https://html.spec.whatwg.org/#htmloutputelement
+
 interface HTMLOutputElement : HTMLElement {
     [PutForwards=value] readonly attribute DOMSettableTokenList htmlFor;
-    [ImplementedAs=formOwner] readonly attribute HTMLFormElement form;
+    [ImplementedAs=formOwner] readonly attribute HTMLFormElement? form;
     [Reflect] attribute DOMString name;
 
     readonly attribute DOMString type;
     attribute DOMString defaultValue;
     attribute DOMString value;
 
     readonly attribute boolean willValidate;
     readonly attribute ValidityState validity;
     readonly attribute DOMString validationMessage;
     boolean checkValidity();
     boolean reportValidity();
+    // TODO(philipj):  The error argument should not be nullable.
     void setCustomValidity([TreatUndefinedAs=NullString] DOMString? error);
 
     readonly attribute NodeList labels;
 };