Implement PermissionDescriptor usage in Permissions API.

Source/modules/permissions/PushPermissionDescriptor.idl

Index: Source/modules/permissions/PushPermissionDescriptor.idl
diff --git a/Source/modules/permissions/PushPermissionDescriptor.idl b/Source/modules/permissions/PushPermissionDescriptor.idl
new file mode 100644
index 0000000000000000000000000000000000000000..d87391a58ae36c200f8fdd6f86f9eb9f6789287b
--- /dev/null
+++ b/Source/modules/permissions/PushPermissionDescriptor.idl
@@ -0,0 +1,8 @@
+// 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.
+
+dictionary PushPermissionDescriptor : PermissionDescriptor {
+    // See https://w3c.github.io/push-api/.
+    boolean userVisible = false;
+};