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

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

<!-- Please describe your changes on the following line: -->

`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

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not require new tests because refactor

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/13134)
<!-- Reviewable:end -->

components/script/dom/cssstyledeclaration.rs

@@ -14,12 +14,13 @@ use dom::element::Element;
 use dom::node::{Node, NodeDamage, window_from_node};
 use dom::window::Window;
 use std::ascii::AsciiExt;
-use std::cell::Ref;
 use std::slice;
+use std::sync::Arc;
 use string_cache::Atom;
 use style::parser::ParserContextExtraData;
 use style::properties::{PropertyDeclaration, Shorthand, Importance};
 use style::properties::{is_supported_property, parse_one_declaration, parse_style_attribute};
+use style::refcell::Ref;
 use style::selector_impl::PseudoElement;
 
 // http://dev.w3.org/csswg/cssom/#the-cssstyledeclaration-interface
@@ -365,7 +366,7 @@ impl CSSStyleDeclarationMethods for CSSStyleDeclaration {
         *element.style_attribute().borrow_mut() = if decl_block.declarations.is_empty() {
             None // Step 2
         } else {
-            Some(decl_block)
+            Some(Arc::new(decl_block))
         };
         element.sync_property_with_attrs_style();
         let node = element.upcast::<Node>();