Disconnect ARC VPN if ARC++ is shut down

Currently, if ARC++ is uninstalled while an Android VPN is running,
shill will not receive a disconnection event and this will break
networking on Chrome OS.  Fix this by forcing ARC VPN to disconnect
whenever ArcNetHostImpl shuts down.

BUG=768053
TEST=uninstall ARC++ through Settings with a VPN connected

Change-Id: Iccbdc20bb26bce154f8041a517b1220519a2b738
Reviewed-on: https://chromium-review.googlesource.com/778340
Commit-Queue: Kevin Cernekee <cernekee@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Elijah Taylor <elijahtaylor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521121}

components/arc/net/arc_net_host_impl.cc

@@ -399,6 +399,10 @@ void ArcNetHostImpl::OnConnectionReady() {
 }
 
 void ArcNetHostImpl::OnConnectionClosed() {
+  // Make sure shill doesn't leave an ARC VPN connected after Android
+  // goes down.
+  AndroidVpnStateChanged(arc::mojom::ConnectionStateType::NOT_CONNECTED);
+
   if (!observing_network_state_)
     return;