Spec: Allow updating to frozen-context executionMode (#953)

spec.bs

@@ -3638,7 +3638,7 @@ The <dfn for=Navigator method>updateAdInterestGroups()</dfn> method steps are:
 
         <dt>"`executionMode`"
         <dd>
-        1. If |value| is "`compatibility`" or "`group-by-origin`",
+        1. If |value| is "`compatibility`", "`frozen-context`", or "`group-by-origin`",
           set |ig|'s [=interest group/execution mode=] to |value|.
         1. Otherwise, jump to the step labeled <i><a href=#abort-update>Abort update</a></i>.
 
@@ -4241,7 +4241,7 @@ An interest group is a [=struct=] with the following [=struct/items=]:
 :: Null or an [=ordered map=] whose [=map/keys=] are [=strings=] and whose [=map/values=] are
   {{double}}. Overrides the {{AuctionAdConfig}}'s corresponding priority signals.
 : <dfn>execution mode</dfn>
-:: "`compatibility`" or "`group-by-origin`".
+:: "`compatibility`", "`frozen-context`", or "`group-by-origin`".
   TODO: Define spec for these execution modes, link to it from here and explain these modes.
 : <dfn>bidding url</dfn>
 :: Null or a [=URL=]. The URL to fetch the buyer's JavaScript from.