Update interfaces/service-workers.idl (#21937)

Source: https://github.com/tidoust/reffy-reports/blob/1849604/ed/idl/service-workers.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/654545578
diff --git a/interfaces/service-workers.idl b/interfaces/service-workers.idl
index c01fcab..3d5e63a 100644
--- a/interfaces/service-workers.idl
+++ b/interfaces/service-workers.idl
@@ -173,6 +173,7 @@
   readonly attribute DOMString clientId;
   readonly attribute DOMString resultingClientId;
   readonly attribute DOMString replacesClientId;
+  readonly attribute Promise<void> handled;
 
   void respondWith(Promise<Response> r);
 };
@@ -183,6 +184,7 @@
   DOMString clientId = "";
   DOMString resultingClientId = "";
   DOMString replacesClientId = "";
+  Promise<void> handled;
 };
 
 [Exposed=ServiceWorker]