`CSSRule::Type()` returns an u16 for CSSOM. `InsertRule()` was
incorrectly using this to create a `CssRuleTypes`.
Instead of `CssRuleTypes::from_bits(rule_type)`, it should be something
like `CssRuleTypes::from_bits(1 << rule_type)`.
However, that would only work when `Type()` provides an actual value,
which per https://drafts.csswg.org/cssom/#dom-cssrule-type only happens
for old rule types. New rule types just return 0.
Therefore, this patch changes the signature of `SpecificCSSRule::ty()`
to return the actual `CssRuleType`, and then `CSSRule::Type()` can zero
it out when necessary.
The fix is only relevant for CSS Nesting, which is currently disabled on
Servo, so no test is necessary.
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relevant for CSS Nesting, which is currently disabled
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