shill: add property change notifications for IPConfig objects

Chrome monitors the kAddressProperty and the kNameServersProperty.
So emit changes for them.

BUG=chromium:230329
TEST=new unit tests

Change-Id: I0015441f62152d3d4c51a46bd95ff73acbf16def
Reviewed-on: https://gerrit.chromium.org/gerrit/48916
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: mukesh agrawal <quiche@chromium.org>
Tested-by: mukesh agrawal <quiche@chromium.org>
diff --git a/mock_ipconfig.h b/mock_ipconfig.h
index 42358c6..ba2f8ff 100644
--- a/mock_ipconfig.h
+++ b/mock_ipconfig.h
@@ -29,6 +29,7 @@
                           const std::string &id_suffix));
   MOCK_METHOD2(Save, bool(StoreInterface *storage,
                           const std::string &id_suffix));
+  MOCK_METHOD0(EmitChanges, void(void));
 
  private:
   const Properties &real_properties() {