Remove the assert_readonly test and add one to verify that assigning to
CSSStyleRule.style correctly forwards to CSSStyleRule.style.cssText.
We currently test for whether CSSStyleRule.style is read-only by
trying to assign to it; however, the spec has it as actually:
interface CSSStyleRule : CSSRule {
...
[SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
};
See: https://drafts.csswg.org/cssom/
The `PutForwards=cssText` means that assigning to CSSStyleRule.style
should actually assign to style.cssText.