Auto merge of #15493 - BorisChiou:animation/get_property, r=Manishearth

stylo: Add Servo_AnimationValue_Serialize

This is the servo-side change for [bug 1337313](https://bugzilla.mozilla.org/show_bug.cgi?id=1337313). @Manishearth had already reviewed it there. Please merge this patch until the gecko-side changes for [bug 1337313](https://bugzilla.mozilla.org/show_bug.cgi?id=1337313) is landed.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [bug 1337313](https://bugzilla.mozilla.org/show_bug.cgi?id=1337313).
- [X] These changes do not require tests because there are existing tests for this in mozilla-central

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components/style/gecko_bindings/bindings.rs

@@ -1303,13 +1303,18 @@ extern "C" {
      -> RawServoDeclarationBlockStrong;
 }
 extern "C" {
-    pub fn Servo_AnimationValues_GetOpacity(value:
-                                                RawServoAnimationValueBorrowed)
+    pub fn Servo_AnimationValue_Serialize(value:
+                                              RawServoAnimationValueBorrowed,
+                                          property: nsCSSPropertyID,
+                                          buffer: *mut nsAString_internal);
+}
+extern "C" {
+    pub fn Servo_AnimationValue_GetOpacity(value: RawServoAnimationValueBorrowed)
      -> f32;
 }
 extern "C" {
-    pub fn Servo_AnimationValues_GetTransform(value: RawServoAnimationValueBorrowed,
-                                              list: &mut RefPtr<nsCSSValueSharedList>);
+    pub fn Servo_AnimationValue_GetTransform(value: RawServoAnimationValueBorrowed,
+                                             list: &mut RefPtr<nsCSSValueSharedList>);
 }
 extern "C" {
     pub fn Servo_ParseStyleAttribute(data: *const nsACString_internal)