auto merge of #3817 : nkdalmia/servo/master, r=jdm

We have completed the initial step for "Implement Window.sessionStorage and Window.localStorage"
- Create and stub the Storage WebIDL interface

components/script/dom/webidls/Storage.webidl

@@ -0,0 +1,25 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * The origin of this IDL file is
+ * https://html.spec.whatwg.org/multipage/webstorage.html#webstorage
+ *
+ */
+
+interface Storage {
+
+  readonly attribute unsigned long length;
+
+  DOMString? key(unsigned long index);
+
+  getter DOMString? getItem(DOMString name);
+
+  //setter creator void setItem(DOMString name, DOMString value);
+
+  //deleter not supported yet
+  //deleter void removeItem(DOMString name);
+
+  void clear();
+};