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/mock_manager.h b/mock_manager.h
index 32f0e45..9b74211 100644
--- a/mock_manager.h
+++ b/mock_manager.h
@@ -23,6 +23,7 @@
   MOCK_METHOD0(device_info, DeviceInfo*(void));
   MOCK_METHOD0(mutable_store, PropertyStore*(void));
   MOCK_CONST_METHOD0(store, const PropertyStore&(void));
+  MOCK_METHOD0(Start, void());
   MOCK_METHOD1(HasService, bool(const ServiceRefPtr &to_manage));
   MOCK_METHOD1(RegisterService, void(const ServiceRefPtr &to_manage));
   MOCK_METHOD1(UpdateService, void(const ServiceRefPtr &to_update));