Service Worker: Include promise types in IDL

Source/modules/serviceworkers/ServiceWorkerRegistration.idl

Index: Source/modules/serviceworkers/ServiceWorkerRegistration.idl
diff --git a/Source/modules/serviceworkers/ServiceWorkerRegistration.idl b/Source/modules/serviceworkers/ServiceWorkerRegistration.idl
index a991c7c9faf8f4422560e79b7ff666b14eeb2e12..0cff7a5c6b26f60009545a0526f10b341854cff0 100644
--- a/Source/modules/serviceworkers/ServiceWorkerRegistration.idl
+++ b/Source/modules/serviceworkers/ServiceWorkerRegistration.idl
@@ -12,9 +12,10 @@
     [Unforgeable] readonly attribute ServiceWorker? installing;
     [Unforgeable] readonly attribute ServiceWorker? waiting;
     [Unforgeable] readonly attribute ServiceWorker? active;
+
     readonly attribute USVString scope;
 
-    [CallWith=ScriptState] Promise unregister();
+    [CallWith=ScriptState] Promise<boolean> unregister();
 
     attribute EventHandler onupdatefound;
 };