Implement PermissionDescriptor usage in Permissions API.

Source/modules/permissions/PermissionDescriptor.idl

Index: Source/modules/permissions/PermissionDescriptor.idl
diff --git a/Source/modules/permissions/PermissionDescriptor.idl b/Source/modules/permissions/PermissionDescriptor.idl
new file mode 100644
index 0000000000000000000000000000000000000000..551a767b1472fbd6a9d59741fe3fcbc69cfe92a2
--- /dev/null
+++ b/Source/modules/permissions/PermissionDescriptor.idl
@@ -0,0 +1,17 @@
+// 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;
+};