tray: Show warning in network tray when VPN or proxy used.

ash/mus/system_tray_delegate_mus.cc

Index: ash/mus/system_tray_delegate_mus.cc
diff --git a/ash/mus/system_tray_delegate_mus.cc b/ash/mus/system_tray_delegate_mus.cc
index dc4a1dc17ab5302a375271cf753b273758beca0a..df6366ac6e2166129de9f06948f1261151e68db9 100644
--- a/ash/mus/system_tray_delegate_mus.cc
+++ b/ash/mus/system_tray_delegate_mus.cc
@@ -25,6 +25,8 @@ class StubNetworkingConfigDelegate : public NetworkingConfigDelegate {
     return nullptr;
   }
 
+  bool HasDefaultNetworkProxyConfigured() override { return false; }
+
   DISALLOW_COPY_AND_ASSIGN(StubNetworkingConfigDelegate);
 };