Use event init dictionaries in modules/{notifications,push_messaging,serviceworkers}

Source/modules/serviceworkers/ExtendableEventInit.idl

Index: Source/modules/serviceworkers/ExtendableEventInit.idl
diff --git a/Source/modules/serviceworkers/RegistrationOptions.idl b/Source/modules/serviceworkers/ExtendableEventInit.idl
similarity index 59%
copy from Source/modules/serviceworkers/RegistrationOptions.idl
copy to Source/modules/serviceworkers/ExtendableEventInit.idl
index cdeda388e49264bc8737fdee2b8e7aea4655688b..81d8afa1eb2fbde77113db0b74b53af134daa234 100644
--- a/Source/modules/serviceworkers/RegistrationOptions.idl
+++ b/Source/modules/serviceworkers/ExtendableEventInit.idl
@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#registration-option-list-dictionary
-dictionary RegistrationOptions {
-    USVString scope = "/";
+// https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#extendable-event-interface
+
+dictionary ExtendableEventInit : EventInit {
+    // Defined for the forward compatibility across the derived events
 };