Cosmetic filtering cannot resolve relative URLs as first-party

[antonok-edm]

CosmeticFiltersJSHandler::OnIsFirstParty contains the following code:

  const auto url = GURL(url_string);
  if (!url.is_valid())
    return false;

This doesn't work when url_string is a relative URL, which always results in an invalid constructed URL. Instead, we should use url_.Resolve(url_string).