Auto merge of #13134 - servo:archery, r=emilio

Add lots of Arc’s in style, and prepare for using DOMRefCell

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`DOMRefCell` usage is not there year because of thread-safety questions, but I have this much already that I’d like to land before it bitrots.

r? @emilio

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require new tests because refactor

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components/style/dom.rs

@@ -195,7 +195,7 @@ pub trait TElement : PartialEq + Debug + Sized + Copy + Clone + ElementExt + Pre
 
     fn as_node(&self) -> Self::ConcreteNode;
 
-    fn style_attribute(&self) -> &Option<PropertyDeclarationBlock>;
+    fn style_attribute(&self) -> Option<&Arc<PropertyDeclarationBlock>>;
 
     fn get_state(&self) -> ElementState;