[CrOS Tether] Add disconnect handling to NetworkConnectionHandler::TetherDelegate.

chromeos/network/network_connect_unittest.cc

Index: chromeos/network/network_connect_unittest.cc
diff --git a/chromeos/network/network_connect_unittest.cc b/chromeos/network/network_connect_unittest.cc
index 2d2d8b7e54e5cfb583746af562d4be0b0120dc36..fccdbc0a270b76ef703a94359d9e142c5c6b9893 100644
--- a/chromeos/network/network_connect_unittest.cc
+++ b/chromeos/network/network_connect_unittest.cc
@@ -75,6 +75,10 @@ class FakeTetherDelegate : public NetworkConnectionHandler::TetherDelegate {
     last_connected_tether_network_guid_ = tether_network_guid;
     success_callback.Run();
   }
+  void DisconnectFromNetwork(
+      const std::string& tether_network_guid,
+      const base::Closure& success_callback,
+      const network_handler::StringResultCallback& error_callback) override {}
 
  private:
   std::string last_connected_tether_network_guid_;