Auto merge of #19035 - nnethercote:bug-1411893, r=emilio

Introduce nsStaticAtom.

It's a sub-class of nsAtom, useful for cases where you know you are dealing
exclusively with static atoms. The nice thing about it is that you can use
raw nsStaticAtom pointers instead of RefPtr<>. (In fact, the AddRef/Release
implementations ensure that we'll crash if we use RefPtr<nsStaticAtom>.)

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- [ ] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1411893

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