Code review comment for lp://staging/~mhr3/unity/home-lens-ordering

Looks good, tests pass. Minor comments:

498 + std::string key_name(key);
Can we get rid of an extra string copy here? I think it would be ok to use glib string comparison.

224 + std::string lens_id(categories_merger_.GetLensIdForCategory(order_vector.at(i)));
Make it const.