Use [Exposed=Window]

whatwg/webidl#365

Author: anssiko

index.html

@@ -566,7 +566,7 @@ <h3>
           historical reasons.
         </div>
         <pre class="idl">
-          [Constructor]
+          [Constructor, Exposed=Window]
           interface BeforeInstallPromptEvent : Event {
               Promise&lt;PromptResponseObject&gt; prompt();
           };