shill: Delete existing routes when the device is started.

BUG=chromium-os:27358
TEST=Unit tests, network_WiFiManager, manually add foreign routes and
ensure they are deleted, also test to make sure default route remains
after DHCP renewal

Change-Id: Ic26b70d685491f31699ff4684ed289e76af8a9ca
Reviewed-on: https://gerrit.chromium.org/gerrit/17640
Commit-Ready: Thieu Le <thieule@chromium.org>
Reviewed-by: Thieu Le <thieule@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>
diff --git a/manager.h b/manager.h
index 80a021e..b04c1b3 100644
--- a/manager.h
+++ b/manager.h
@@ -57,7 +57,7 @@
 
   void AddDeviceToBlackList(const std::string &device_name);
 
-  void Start();
+  virtual void Start();
   void Stop();
 
   const ProfileRefPtr &ActiveProfile() const;