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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---
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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)

ports/geckolib/glue.rs

@@ -213,8 +213,33 @@ pub extern "C" fn Servo_AnimationValues_Uncompute(value: RawServoAnimationValueB
     })).into_strong()
 }
 
+macro_rules! get_property_id_from_nscsspropertyid {
+    ($property_id: ident, $ret: expr) => {{
+        match PropertyId::from_nscsspropertyid($property_id) {
+            Ok(property_id) => property_id,
+            Err(()) => { return $ret; }
+        }
+    }}
+}
+
+#[no_mangle]
+pub extern "C" fn Servo_AnimationValue_Serialize(value: RawServoAnimationValueBorrowed,
+                                                 property: nsCSSPropertyID,
+                                                 buffer: *mut nsAString)
+{
+    let uncomputed_value = AnimationValue::as_arc(&value).uncompute();
+    let mut string = String::new();
+    let rv = PropertyDeclarationBlock {
+        declarations: vec![(uncomputed_value, Importance::Normal)],
+        important_count: 0
+    }.single_value_to_css(&get_property_id_from_nscsspropertyid!(property, ()), &mut string);
+    debug_assert!(rv.is_ok());
+
+    write!(unsafe { &mut *buffer }, "{}", string).expect("Failed to copy string");
+}
+
 #[no_mangle]
-pub extern "C" fn Servo_AnimationValues_GetOpacity(value: RawServoAnimationValueBorrowed)
+pub extern "C" fn Servo_AnimationValue_GetOpacity(value: RawServoAnimationValueBorrowed)
      -> f32
 {
     let value = AnimationValue::as_arc(&value);
@@ -226,8 +251,8 @@ pub extern "C" fn Servo_AnimationValues_GetOpacity(value: RawServoAnimationValue
 }
 
 #[no_mangle]
-pub extern "C" fn Servo_AnimationValues_GetTransform(value: RawServoAnimationValueBorrowed,
-                                                     list: &mut structs::RefPtr<nsCSSValueSharedList>)
+pub extern "C" fn Servo_AnimationValue_GetTransform(value: RawServoAnimationValueBorrowed,
+                                                    list: &mut structs::RefPtr<nsCSSValueSharedList>)
 {
     let value = AnimationValue::as_arc(&value);
     if let AnimationValue::Transform(ref servo_list) = **value {
@@ -832,15 +857,6 @@ pub extern "C" fn Servo_DeclarationBlock_GetCssText(declarations: RawServoDeclar
     declarations.read().to_css(unsafe { result.as_mut().unwrap() }).unwrap();
 }
 
-macro_rules! get_property_id_from_nscsspropertyid {
-    ($property_id: ident, $ret: expr) => {{
-        match PropertyId::from_nscsspropertyid($property_id) {
-            Ok(property_id) => property_id,
-            Err(()) => { return $ret; }
-        }
-    }}
-}
-
 #[no_mangle]
 pub extern "C" fn Servo_DeclarationBlock_SerializeOneValue(
     declarations: RawServoDeclarationBlockBorrowed,