Auto merge of #15695 - canaltinova:stylo-font-face, r=Manishearth

Stylo: Add constructors for @font-face descriptors

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Part of https://bugzilla.mozilla.org/show_bug.cgi?id=1340728

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug #1340728](https://bugzilla.mozilla.org/show_bug.cgi?id=1340728)

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/style/gecko_bindings/bindings.rs

@@ -840,6 +840,26 @@ extern "C" {
     pub fn Gecko_CSSValue_SetFunction(css_value: nsCSSValueBorrowedMut,
                                       len: i32);
 }
+extern "C" {
+    pub fn Gecko_CSSValue_SetString(css_value: nsCSSValueBorrowedMut,
+                                    string: nsString);
+}
+extern "C" {
+    pub fn Gecko_CSSValue_SetArray(css_value: nsCSSValueBorrowedMut,
+                                   len: i32);
+}
+extern "C" {
+    pub fn Gecko_CSSValue_SetURL(css_value: nsCSSValueBorrowedMut,
+                                 uri: ServoBundledURI);
+}
+extern "C" {
+    pub fn Gecko_CSSValue_SetLocal(css_value: nsCSSValueBorrowedMut,
+                                   family: nsString);
+}
+extern "C" {
+    pub fn Gecko_CSSValue_SetInteger(css_value: nsCSSValueBorrowedMut,
+                                     integer: i32);
+}
 extern "C" {
     pub fn Gecko_CSSValue_Drop(css_value: nsCSSValueBorrowedMut);
 }