Auto merge of #18722 - heycam:family-list, r=upsuper

style: Use a SharedFontList object to store font-family values for Gecko

Reviewed by Xidorn and Manish in https://bugzilla.mozilla.org/show_bug.cgi?id=1397626.

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components/gfx/font_context.rs

@@ -135,7 +135,7 @@ impl FontContext {
 
         let mut fonts: SmallVec<[Rc<RefCell<Font>>; 8]> = SmallVec::new();
 
-        for family in &style.font_family.0 {
+        for family in style.font_family.0.iter() {
             // GWTODO: Check on real pages if this is faster as Vec() or HashMap().
             let mut cache_hit = false;
             for cached_font_entry in &self.layout_font_cache {