servo: Merge #15695 - Stylo: Add constructors for @font-face descriptors (from canaltinova:stylo-font-face); r=Manishearth

<!-- Please describe your changes on the following line: -->
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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Source-Repo: https://github.com/servo/servo
Source-Revision: ad2ca516a2b3c81f577a0b750cdda12058006da1

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : a7b666ce553b3fcd6e55af2f421b6c62768164b9

servo/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);
 }