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/script/dom/document.rs

@@ -111,7 +111,7 @@ use script_traits::{TouchEventType, TouchId};
 use std::ascii::AsciiExt;
 use std::borrow::ToOwned;
 use std::boxed::FnBox;
-use std::cell::{Cell, Ref, RefMut};
+use std::cell::Cell;
 use std::collections::HashMap;
 use std::collections::hash_map::Entry::{Occupied, Vacant};
 use std::default::Default;
@@ -122,6 +122,7 @@ use std::sync::Arc;
 use string_cache::{Atom, QualName};
 use style::attr::AttrValue;
 use style::context::ReflowGoal;
+use style::refcell::{Ref, RefMut};
 use style::selector_impl::ElementSnapshot;
 use style::str::{split_html_space_chars, str_join};
 use style::stylesheets::Stylesheet;