Auto merge of #17404 - zuwow:shrink-rulenode, r=bholley

Shrink rulenode

This shrinks rulenode by one word by removing the Option wrapper around the StyleSource and adding None to StyleSource as an additional variant. The issue mentions shrinking by two words but the free_count one was taken care of in #17368.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #17280

Also tested with `./mach test-unit` and `./mach test-stylo` with no errors reported from either.

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- [X] There are tests for these changes

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r? @bholley

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tests/unit/stylo/size_of.rs

@@ -42,7 +42,7 @@ size_of_test!(test_size_of_application_declaration_block, ApplicableDeclarationB
 
 // FIXME(bholley): This can shrink with a little bit of work.
 // See https://github.com/servo/servo/issues/17280
-size_of_test!(test_size_of_rule_node, RuleNode, 88);
+size_of_test!(test_size_of_rule_node, RuleNode, 80);
 
 // This is huge, but we allocate it on the stack and then never move it,
 // we only pass `&mut SourcePropertyDeclaration` references around.