Auto merge of #16224 - servo:valid-fontface, r=upsuper

Make the parser accept @font-face rules without font-family or src.

Fix #16165.

Also, it turns out that the CSSFontFaceRule IDL specified in the css-fonts spec is not web-compatible. Instead browsers implement a .style attribute like in CSSStyleRule: https://github.com/w3c/csswg-drafts/issues/825

This in turn requires preserving data about which descriptors were set or not (distinguishing unset from set to a value that happens to be the initial value), so this commit also makes every field `Option<_>`.

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components/style/gecko_bindings/sugar/mod.rs

@@ -6,7 +6,7 @@
 
 mod ns_com_ptr;
 mod ns_css_shadow_array;
-mod ns_css_value;
+pub mod ns_css_value;
 mod ns_style_auto_array;
 pub mod ns_style_coord;
 mod ns_t_array;

components/style/gecko_bindings/sugar/ns_css_value.rs

@@ -163,6 +163,11 @@ impl nsCSSValue {
         unsafe { bindings::Gecko_CSSValue_SetArray(self, len) }
         unsafe { self.mValue.mArray.as_mut().as_mut() }.unwrap()
     }
+
+    /// Generic set from any value that implements the ToNsCssValue trait.
+    pub fn set_from<T: ToNsCssValue>(&mut self, value: &T) {
+        value.convert(self)
+    }
 }
 
 impl Drop for nsCSSValue {
@@ -210,3 +215,9 @@ impl IndexMut<usize> for nsCSSValue_Array {
         &mut self.as_mut_slice()[i]
     }
 }
+
+/// Generic conversion to nsCSSValue
+pub trait ToNsCssValue {
+    /// Convert
+    fn convert(&self, nscssvalue: &mut nsCSSValue);
+}