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.

---
<!-- 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
- [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/python/tidy/servo_tidy/licenseck.py

@@ -69,5 +69,16 @@ licenses = [
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
-"""
+""",
+
+"""\
+// Copyright 2012-2014 The Rust Project Developers.
+// See http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+""",
 ]  # noqa: Indicate to flake8 that we do not want to check indentation here