Make CSSStyleRule inherit from CSSGroupingRule.

As per w3c/csswg-drafts#8940.

I didn't do this in bug 1837638 because that's what the spec said at the
time, that's what other browsers did, and specially because if we did
this we had no way of runtime-disable nesting during development or if
things went south.

This means that we can't keep the nesting pref in 118, but that seems
fine (it's already enabled everywhere in 117).

Differential Revision: https://phabricator.services.mozilla.com/D184930

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1846251
gecko-commit: 62534bac923526d52c2d5f377d00bdf2d34b9a05
gecko-reviewers: peterv, devtools-reviewers

Author: emilio

css/css-nesting/cssom.html

@@ -8,6 +8,10 @@
 <style id="ss"></style>
 
 <script>
+  test(() => {
+    assert_equals(CSSStyleRule.__proto__, CSSGroupingRule);
+  }, "CSSStyleRule is a CSSGroupingRule");
+
   test(() => {
     let [ss] = document.styleSheets;
     assert_equals(ss.cssRules.length, 0);

interfaces/cssom.idl

@@ -96,12 +96,6 @@ interface CSSRule {
   const unsigned short NAMESPACE_RULE = 10;
 };
 
-[Exposed=Window]
-interface CSSStyleRule : CSSRule {
-  attribute CSSOMString selectorText;
-  [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
-};
-
 [Exposed=Window]
 interface CSSImportRule : CSSRule {
   readonly attribute USVString href;
@@ -118,6 +112,12 @@ interface CSSGroupingRule : CSSRule {
   undefined deleteRule(unsigned long index);
 };
 
+[Exposed=Window]
+interface CSSStyleRule : CSSGroupingRule {
+  attribute CSSOMString selectorText;
+  [SameObject, PutForwards=cssText] readonly attribute CSSStyleDeclaration style;
+};
+
 [Exposed=Window]
 interface CSSPageRule : CSSGroupingRule {
            attribute CSSOMString selectorText;