Define push_messaging module with PushManager skeleton.

Source/modules/push_messaging/NavigatorPushManager.idl

Index: Source/modules/push_messaging/NavigatorPushManager.idl
diff --git a/Source/modules/push_messaging/NavigatorPushManager.idl b/Source/modules/push_messaging/NavigatorPushManager.idl
new file mode 100644
index 0000000000000000000000000000000000000000..26db0dddc171d946dab70ea59e2202a465f47803
--- /dev/null
+++ b/Source/modules/push_messaging/NavigatorPushManager.idl
@@ -0,0 +1,9 @@
+// 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.
+
+[
+    RuntimeEnabled=PushMessaging,
+] partial interface Navigator {
+    readonly attribute PushManager push;
+};