Service Worker: Include promise types in IDL

Source/modules/serviceworkers/Body.idl

 // 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://fetch.spec.whatwg.org/#body
+typedef object JSON;
 [
     RuntimeEnabled=ServiceWorker,
     Exposed=ServiceWorker,
     NoInterfaceObject,
     ActiveDOMObject,
     GarbageCollected,
     TypeChecking=Interface,
 ] interface Body {
     readonly attribute boolean bodyUsed;
-    [CallWith=ScriptState] Promise arrayBuffer();
-    [CallWith=ScriptState] Promise blob();
-    [CallWith=ScriptState] Promise json();
-    [CallWith=ScriptState] Promise text();
+    [CallWith=ScriptState] Promise<ArrayBuffer> arrayBuffer();
+    [CallWith=ScriptState] Promise<Blob> blob();
+    [CallWith=ScriptState] Promise<JSON> json();
+    [CallWith=ScriptState] Promise<USVString> text();
 
     // Still to be implemented.
-    // [CallWith=ScriptState] Promise formData();
+    // [CallWith=ScriptState] Promise<FormData> formData();
 };