Export fetch controller/serialized abort reason

This is referenced [1] by the Service Worker spec, so should either be exported or something else to achieve the same thing would need to exist (forwarding the "abort reason" from a fetch controller to an AbortController; note that the service worker algorithm is currently extra confusing because it uses the same variable name for two different objects).

[1]: https://w3c.github.io/ServiceWorker/#clients-openwindow:~:text=If%20controller%E2%80%99s%20serialized%20abort%20reason%20is%20non%2Dnull

fetch.bs

@@ -236,7 +236,7 @@ lt="authentication entry">authentication entries</a> (for HTTP authentication).
  <dt><dfn for="fetch controller">report timing steps</dfn> (default null)
  <dd>Null or an algorithm accepting a <a for=/>global object</a>.
 
- <dt><dfn for="fetch controller">serialized abort reason</dfn> (default null)
+ <dt><dfn for="fetch controller" export>serialized abort reason</dfn> (default null)
  <dd>Null or a <a>Record</a> (result of [$StructuredSerialize$]).
 
  <dt><dfn for="fetch controller">next manual redirect steps</dfn> (default null)