shill: Adds OnNewFamilyMessage tests to netlink_manager_unittest.

This required extending some mocks.

BUG=chromium:222486
TEST=unittest

Change-Id: I737be51b288cb2fe26a71708e077e9c3c917910e
Reviewed-on: https://gerrit.chromium.org/gerrit/55571
Commit-Queue: Wade Guthrie <wdg@chromium.org>
Reviewed-by: Wade Guthrie <wdg@chromium.org>
Tested-by: Wade Guthrie <wdg@chromium.org>
diff --git a/mock_netlink_manager.h b/mock_netlink_manager.h
index e88d33e..27697af 100644
--- a/mock_netlink_manager.h
+++ b/mock_netlink_manager.h
@@ -19,6 +19,11 @@
  public:
   MockNetlinkManager();
   virtual ~MockNetlinkManager();
+
+  MOCK_METHOD2(
+      GetFamily,
+      uint16_t(const std::string &family_name,
+               const NetlinkMessageFactory::FactoryMethod &message_factory));
   MOCK_METHOD1(RemoveBroadcastHandler,
                bool(const NetlinkMessageHandler &message_handler));
   MOCK_METHOD1(AddBroadcastHandler,