shill: implement Service.Disconnect (for WiFiService only)

BUG=chromium-os:23774,chromium-os:23662
TEST=new unit tests, some autotests

autotests run:
- PASS network_WiFiSecMat.031CheckHidden_WPA
- PASS network_WiFiSecMat.032CheckHidden_RSN

Change-Id: I0c2dc8c9888caca793f0dfe5a55c0df2a4b10c79
Reviewed-on: https://gerrit.chromium.org/gerrit/12498
Commit-Ready: mukesh agrawal <quiche@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: mukesh agrawal <quiche@chromium.org>
diff --git a/cellular_service.cc b/cellular_service.cc
index 2daf880..b7b3b8a 100644
--- a/cellular_service.cc
+++ b/cellular_service.cc
@@ -49,7 +49,7 @@
   cellular_->Connect(error);
 }
 
-void CellularService::Disconnect() { }
+void CellularService::Disconnect(Error */*error*/) { }
 
 void CellularService::ActivateCellularModem(const string &carrier,
                                             Error *error) {