Implement PermissionDescriptor usage in Permissions API.

Source/modules/permissions/PermissionDescriptor.idl

+// Copyright 2015 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.
+
+enum PermissionName {
+    "geolocation",
+    "midi",
+    "notifications",
+    "push"
+};
+
+// The PermissionDescriptor dictionary is a base to describe permissions. Some
+// permissions will extend it. The methods reading it will re-parse it depending
+// on the name.
+dictionary PermissionDescriptor {
+    /* required */ PermissionName name;
+};