Track stylesheet load's document instead of using element's current document
For cases where a stylesheet load finishes in a different document than it started, we need to be more careful about which document we report the completion to. In this case we actually have separate requests for each document involved, but they previously used the same element to determine which document to interact with.
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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#14641
- [X] There are tests for these changes OR
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Reimplement AtomicRefCell with pure atomics
While reviewing @bzbarsky's patches in [1], I started typing out some lore about how mutable AtomicRefCell borrows are actually cheaper than immutable ones, so we should prefer them where possible. But then I decided that this was a really dumb state of affairs and that we should just fix AtomicRefCell instead, and implement a proper AtomicRef{,Mut}::map while we were at it. So here we are.
This PR adds a from-scratch implementation of AtomicRefCell that aims to be 100% sound, even in unrealistic overflow scenarios. We should probably get this on crates.io eventually, but I want to land it landed in-tree first.
With this implementation, each operation (borrow or release) is one atomic instruction, and all borrow/release pairs (mutable or immutable) take 12 ns on my machine, which is what I'd expect. This is a 50% improvement over the previous implementation in the immutable case.
There may be some places where we could get away with Ordering::Release instead of Ordering::AcqRel, but it didn't seem worth it to try to reason it out.
r? @Manishearth
CC @emilio @SimonSapin @heycam @upsuper
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1298588
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