for_first_not_loaded_import_rule()
This was only used for making sure `@import` rules got loaded, and since they handle that themselves now, we can get rid of all this code!
The logic is handled by `CSSGroupingRule` and `CSSConditionRule`, so `CSSMediaRule` only has to report if its condition matches. Right now, that condition is always false because we do not evaluate the media query.
https://www.w3.org/TR/css-conditional-3/#the-cssconditionrule-interface This simply exposes a condition string, which is implemented differently in each sub-class.