turn assert into promise rejection

spec.bs

@@ -1836,11 +1836,9 @@ Several APIs specific to fenced frames are defined on the {{Fence}} interface.
 
   1. If |instance| is null, then [=resolve=] |p| with {{undefined}} and return |p|.
 
-  1. [=Assert=]: the [=relevant settings object=]'s [=environment settings object/origin=] and
-     |instance|'s [=fenced frame config instance/mapped url=]'s [=url/origin=] are [=same origin=].
-
-     Note: Only contexts that are [=same origin=] to the [=fenced frame config instance/mapped url=]
-     described in the [=fenced frame config instance=] are allowed to call this API.
+  1. If the [=relevant settings object=]'s [=environment settings object/origin=] and
+     |instance|'s [=fenced frame config instance/mapped url=]'s [=url/origin=] are not [=same
+     origin=], then [=reject=] |p| with a {{TypeError}}.
 
   1. If [=this=]'s [=relevant global object=]'s [=Window/navigable=]'s [=navigable/traversable
      navigable=] is not a [=fenced navigable container/fenced navigable=], then [=resolve=] |p| with