Push API: rename registration to subscription.

Source/modules/push_messaging/PushRegistration.idl

Index: Source/modules/push_messaging/PushRegistration.idl
diff --git a/Source/modules/push_messaging/PushRegistration.idl b/Source/modules/push_messaging/PushRegistration.idl
deleted file mode 100644
index 243215d9fc20fb200d414d64097ca52d7a21f3df..0000000000000000000000000000000000000000
--- a/Source/modules/push_messaging/PushRegistration.idl
+++ /dev/null
@@ -1,14 +0,0 @@
-// Copyright 2014 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-// http://www.w3.org/TR/push-api/#pushregistration-interface
-
-[
-    GarbageCollected,
-    RuntimeEnabled=PushMessaging,
-] interface PushRegistration {
-    readonly attribute DOMString endpoint;
-    readonly attribute DOMString registrationId;
-    [CallWith=ScriptState] Promise<boolean> unregister();
-};