Export the Clear Registration Algorithm

So that the Clear Site Data spec can reference the algorithm and use
it to clear out the service worker, insteaed of unregister().

Context:
w3c#614 (comment)

Eventually will resolve:
w3c/webappsec-clear-site-data#54

docs/index.bs

@@ -3086,7 +3086,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
   </section>
 
   <section algorithm>
-    <h3 id="clear-registration-algorithm"><dfn>Clear Registration</dfn></h3>
+    <h3 id="clear-registration-algorithm"><dfn export>Clear Registration</dfn></h3>
 
       : Input
       :: |registration|, a [=/service worker registration=]