Export ServiceWorkerGlobalScope's service worker for DOM Standards (#1476)

whatwg/dom#653 needs a reference to this dfn.

Author: jungkees

docs/index.bs

@@ -1000,7 +1000,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
       };
     </pre>
 
-    A {{ServiceWorkerGlobalScope}} object represents the global execution context of a [=/service worker=]. A {{ServiceWorkerGlobalScope}} object has an associated <dfn for="ServiceWorkerGlobalScope">service worker</dfn> (a [=/service worker=]). A {{ServiceWorkerGlobalScope}} object has an associated <dfn for="ServiceWorkerGlobalScope">force bypass cache for import scripts flag</dfn>. It is initially unset.
+    A {{ServiceWorkerGlobalScope}} object represents the global execution context of a [=/service worker=]. A {{ServiceWorkerGlobalScope}} object has an associated <dfn export for="ServiceWorkerGlobalScope">service worker</dfn> (a [=/service worker=]). A {{ServiceWorkerGlobalScope}} object has an associated <dfn for="ServiceWorkerGlobalScope">force bypass cache for import scripts flag</dfn>. It is initially unset.
 
     Note: {{ServiceWorkerGlobalScope}} object provides generic, event-driven, time-limited script execution contexts that run at an origin. Once successfully <a>registered</a>, a [=/service worker=] is started, kept alive and killed by their relationship to events, not [=/service worker clients=]. Any type of synchronous requests must not be initiated inside of a [=/service worker=].