Revert of chromeos: Convert ash VPNDelegate interface to mojo

ash/common/system/tray/system_tray_delegate.cc

Index: ash/common/system/tray/system_tray_delegate.cc
diff --git a/ash/common/system/tray/system_tray_delegate.cc b/ash/common/system/tray/system_tray_delegate.cc
index a028b891433b361ff0ea58d8184281a56c5e7d2c..0b6510f7121d93f6e6d94bb2c9e7f272f2bcdcde 100644
--- a/ash/common/system/tray/system_tray_delegate.cc
+++ b/ash/common/system/tray/system_tray_delegate.cc
@@ -158,6 +158,10 @@
 void SystemTrayDelegate::RemoveCustodianInfoTrayObserver(
     CustodianInfoTrayObserver* observer) {}
 
+VPNDelegate* SystemTrayDelegate::GetVPNDelegate() const {
+  return nullptr;
+}
+
 std::unique_ptr<SystemTrayItem> SystemTrayDelegate::CreateRotationLockTrayItem(
     SystemTray* tray) {
   return nullptr;