Auto merge of #24500 - kitlith:style_cfg, r=jdm

Remove usage of opts::get() from style.

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Use `AtomicBool`s to create a global storage for a couple of settings used by an implementation of `Default`, suggested in https://github.com/servo/servo/issues/22854#issuecomment-540955207.

I do have one main question: I'm currently setting these statics in `Servo::new()`, would there be a better place to put this? Same goes with making these `pub`, is there a better way to go about this?

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- [X] `./mach build -d` does not report any errors
  - Seems to be failing in script for unknown/unrelated reasons, experiencing SIGKILL on two different machines. (May be an issue with the latest commit?) Still works fine on CI though.
- [X] `./mach test-tidy` does not report any errors
- [X] These changes help with #22854

These changes do not require tests because these are cleanup changes.

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