Let static prefs default to style_config's prefs

[Loirooriol]

This will allow Servo to enable them e.g. when turning on experimental features (--enable-experimental-web-platform-features), or when running specific tests.

Servo PR: servo/servo#42410

[nicoburns]

Hmm... isn't the point of static prefs that they compile out entirely. If we're doing a runtime pref then that should probably be an explicit calls into style_config from the style crate?

[Loirooriol]

Well, Firefox uses static_prefs::pref!() and you can change prefs during runtime.

[Loirooriol]

The "static" part is indeed confusing if they aren't really static... But apparently in Firefox static_prefs is just what exposes Gecko prefs to rust.

[nicoburns]

Oh huh. I see that gecko doesn't have an equivalent to style_config at all (it just uses it's equivalent to static_prefs). In that case I have no objection We could even consider merging our two config crates.

[Loirooriol]

Yeah, I guess the crates should be merged, and most of the prefs that we are setting in stylo_static_prefs/src/lib.rs should be set from Servo (or Blitz or whatever). But can be left for follow-ups.