Bug 1332633 - Part 2: Implement the FFI for compute distance.

This FFI will be used by nsDOMWindowUtils::ComputeAnimationDistance,
which will be implemented in another bug.

MozReview-Commit-ID: 6KmMmRiYIew

components/style/gecko_bindings/bindings.rs

@@ -1753,6 +1753,13 @@ extern "C" {
                                                     RawServoAnimationValueBorrowed)
      -> bool;
 }
+extern "C" {
+    pub fn Servo_AnimationValues_ComputeDistance(from:
+                                                     RawServoAnimationValueBorrowed,
+                                                 to:
+                                                     RawServoAnimationValueBorrowed)
+     -> f64;
+}
 extern "C" {
     pub fn Servo_AnimationValue_Serialize(value:
                                               RawServoAnimationValueBorrowed,

ports/geckolib/glue.rs

@@ -74,7 +74,7 @@ use style::parser::{LengthParsingMode, ParserContext};
 use style::properties::{CascadeFlags, ComputedValues, Importance, ParsedDeclaration};
 use style::properties::{PropertyDeclarationBlock, PropertyId};
 use style::properties::SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP;
-use style::properties::animated_properties::{AnimationValue, Interpolate, TransitionProperty};
+use style::properties::animated_properties::{AnimationValue, ComputeDistance, Interpolate, TransitionProperty};
 use style::properties::parse_one_declaration;
 use style::restyle_hints::{self, RestyleHint};
 use style::rule_tree::StyleSource;
@@ -265,6 +265,15 @@ pub extern "C" fn Servo_AnimationValues_IsInterpolable(from: RawServoAnimationVa
     from_value.interpolate(to_value, 0.5).is_ok()
 }
 
+#[no_mangle]
+pub extern "C" fn Servo_AnimationValues_ComputeDistance(from: RawServoAnimationValueBorrowed,
+                                                        to: RawServoAnimationValueBorrowed)
+                                                        -> f64 {
+    let from_value = AnimationValue::as_arc(&from);
+    let to_value = AnimationValue::as_arc(&to);
+    from_value.compute_distance(to_value).unwrap_or(0.0)
+}
+
 #[no_mangle]
 pub extern "C" fn Servo_AnimationValueMap_Push(value_map: RawServoAnimationValueMapBorrowed,
                                                property: nsCSSPropertyID,