[Spec] Disallow calling registerAdBeacon for undefined reserved.* typ…

…es (WICG#968)

* [Spec] Disallow calling registerAdBeacon for undefined reserved.* types

* remove manual export

* address review comments

spec.bs

@@ -3515,7 +3515,10 @@ Each {{InterestGroupReportingScriptRunnerGlobalScope}} has a
   1. If [=this=]'s [=relevant global object=]'s [=InterestGroupReportingScriptRunnerGlobalScope/
      reporting beacon map=] is not null, then [=exception/Throw=] a {{TypeError}}.
 
-  1. [=map/For each=] |url| of |map|'s [=map/values=]:
+  1. [=map/For each=] |type| → |url| of |map|:
+
+     1. If |type| [=string/starts with=] "`reserved.`" and |type| does not match one of the
+        [=fencedframetype/automatic beacon event type=] values, [=exception/throw=] a {{TypeError}}.
 
      1. Let |parsedURL| be the result of running [=URL parser=] on |url|.