Meta: export fetch controller/serialized abort reason

This is referenced by Service Workers.

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)