shill: add disconnect reason to log messages

Allow logging of function/reason for triggering disconnect.

BUG=chromium:227295
TEST=unit tests

Change-Id: I92ea3cca664c060bc02bbb8d50d303ce11d1a7de
Reviewed-on: https://chromium-review.googlesource.com/207020
Tested-by: Samuel Tan <samueltan@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Samuel Tan <samueltan@chromium.org>
diff --git a/vpn_service.h b/vpn_service.h
index 50e0614..3ac9171 100644
--- a/vpn_service.h
+++ b/vpn_service.h
@@ -29,7 +29,7 @@
 
   // Inherited from Service.
   virtual void Connect(Error *error, const char *reason);
-  virtual void Disconnect(Error *error);
+  virtual void Disconnect(Error *error, const char *reason);
   virtual std::string GetStorageIdentifier() const;
   virtual bool Load(StoreInterface *storage);
   virtual bool Save(StoreInterface *storage);