Auto merge of #18569 - mrbkap:1398393-cooperative-thread-tagging, r=emilio

Bug 1398393 - Add a way to initialize a cooperative thread in servo.

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Over in [bug 1398393](https://bugzilla.mozilla.org/show_bug.cgi?id=1398393), I found that when we turn Quantum DOM's cooperative threads on, we hit a Servo panic because we assume that we've set some state in the TLD. The cooperative threads currently have no way of doing this without also re-initializing the log module and some other static data. This small patch gives the cooperative threads a way of informing Servo about themselves.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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- [X] These changes do not require tests because they will be tested on Gecko's tinderbox once we get cooperative threading working.

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