In `cxx20`, there is a new function in associative containers called `contains`, which should be used as a replacement to `base::Contains`. This reland makes use of a `base::flat_set<ContentGroup>` in one test, as this was causing build failures on x86 builds, as the compiler would not abide to evaluate `MakeFixedFlatSet` as `constexpr`. Resolves https://github.com/brave/brave-browser/issues/44063