Make the formdata event bubble

This is a follow-up of #4239, and
fixes an issue pointed out in web-platform-tests/wpt#14637.

source

@@ -56651,7 +56651,8 @@ fur
    <li><p><span data-x="concept-event-fire">Fire an event</span> named
    <code data-x="event-formdata">formdata</code> at <var>form</var> using
    <code>FormDataEvent</code>, with the <code data-x="dom-FormDataEvent-formData">formData</code>
-   attribute initialized to <var>form data</var>.</p></li>
+   attribute initialized to <var>form data</var> and the
+   <code data-x="dom-Event-bubbles">bubbles</code> attribute initialized to true.</p></li>
 
    <li><p>Set <var>form</var>'s <span>constructing entry list</span> to false.</p></li>