style: Use Atomic<bool> for the staticpref version of layout.css.font-variations.enabled.

Atomic<bool> is implemented in terms of AtomicBase<uint32_t>, because that way
you don't need to depend on atomic 1-byte operations.  That means that the rust
bindgen sees it as a u32, not a bool.

It's a bit concerning that the rust code seems to be doing an unsynchronized
read here, but given this is a RelaxedAtomic, that's probably ok.

Bug: 1467134
Reviewed-by: emilio

components/style/font_face.rs

@@ -323,7 +323,7 @@ macro_rules! is_descriptor_enabled {
     ("font-variation-settings") => {
         unsafe {
             use gecko_bindings::structs::mozilla;
-            mozilla::StaticPrefs_sVarCache_layout_css_font_variations_enabled
+            mozilla::StaticPrefs_sVarCache_layout_css_font_variations_enabled != 0
         }
     };
     ($name:tt) => {