Add ash::VPNDelegate and Chrome OS implementation

ash/system/tray/system_tray_delegate.cc

 // Copyright (c) 2012 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.
 
 #include "ash/system/tray/system_tray_delegate.h"
 
 namespace ash {
 
 NetworkIconInfo::NetworkIconInfo()
     : connecting(false),
@@ skipping 264 matching lines @@
 }
 
 void SystemTrayDelegate::RemoveShutdownPolicyObserver(
     ShutdownPolicyObserver* observer) {
 }
 
 void SystemTrayDelegate::ShouldRebootOnShutdown(
     const RebootOnShutdownCallback& callback) {
 }
 
+VPNDelegate* SystemTrayDelegate::GetVPNDelegate() const {
+  return nullptr;
+}
+
 }  // namespace ash