Bug 1311257 - Add an FFI function that returns an AnimationValue for a given nsCSSPropertyID from computed values. r=heycam

When we compose style for a given nsCSSPropertyID in the case where
we have no specified values in target keyframe, we use this AnimationValue
for composition as if it's specified.

MozReview-Commit-ID: 2CTLIGgeLIO

--HG--
extra : rebase_source : 3be5bdaf3d226ff004bb0af0f832e5af88f15b6d

layout/style/ServoBindingList.h

@@ -133,6 +133,10 @@ SERVO_BINDING_FUNC(Servo_AnimationValueMap_Push, void,
                    RawServoAnimationValueMapBorrowed,
                    nsCSSPropertyID property,
                    RawServoAnimationValueBorrowed value)
+SERVO_BINDING_FUNC(Servo_ComputedValues_ExtractAnimationValue,
+                   RawServoAnimationValueStrong,
+                   ServoComputedValuesBorrowed computed_values,
+                   nsCSSPropertyID property)
 
 // AnimationValues handling
 SERVO_BINDING_FUNC(Servo_AnimationValues_Interpolate,