servo: Merge #11835 - Use our copy of RefCell for style data (from servo:diy-as-unsafe-cell); r=nox

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This allows removing `#![feature(as_unsafe_cell)]` in geckolib and make progress towards #11815.

---
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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 part of #11815.

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- [ ] There are tests for these changes OR
- [x] These changes do not require new tests because they don’t introduce no functional changes.

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Source-Repo: https://github.com/servo/servo
Source-Revision: 3de14d9ca0cf54e6a2ae4eac5ff47e38e724b77b

servo/components/script/layout_wrapper.rs

@@ -50,7 +50,6 @@ use script_layout_interface::{OpaqueStyleAndLayoutData, PartialStyleAndLayoutDat
 use selectors::matching::{DeclarationBlock, ElementFlags};
 use selectors::parser::{AttrSelector, NamespaceConstraint};
 use smallvec::VecLike;
-use std::cell::{Ref, RefCell, RefMut};
 use std::marker::PhantomData;
 use std::mem::{transmute, transmute_copy};
 use string_cache::{Atom, BorrowedAtom, BorrowedNamespace, Namespace};
@@ -60,6 +59,7 @@ use style::dom::{PresentationalHintsSynthetizer, OpaqueNode, TDocument, TElement
 use style::element_state::*;
 use style::properties::{ComputedValues, ServoComputedValues};
 use style::properties::{PropertyDeclaration, PropertyDeclarationBlock};
+use style::refcell::{Ref, RefCell, RefMut};
 use style::restyle_hints::ElementSnapshot;
 use style::selector_impl::{NonTSPseudoClass, ServoSelectorImpl};
 use style::servo::{PrivateStyleData, SharedStyleContext};