Web Animations: define distance for AnimatableColor

Source/core/animation/AnimatableValue.h

Index: Source/core/animation/AnimatableValue.h
diff --git a/Source/core/animation/AnimatableValue.h b/Source/core/animation/AnimatableValue.h
index 3771fb4354dba85e61731320be796f5fff54a9d1..de043f52914d008fa49735beb49afe5c769ffdbd 100644
--- a/Source/core/animation/AnimatableValue.h
+++ b/Source/core/animation/AnimatableValue.h
@@ -46,6 +46,7 @@ public:
     static PassRefPtr<AnimatableValue> interpolate(const AnimatableValue*, const AnimatableValue*, double fraction);
     // For noncommutative values read add(A, B) to mean the value A with B composed onto it.
     static PassRefPtr<AnimatableValue> add(const AnimatableValue*, const AnimatableValue*);
+    static double distance(const AnimatableValue* from, const AnimatableValue* to);
     static bool usesDefaultInterpolation(const AnimatableValue* from, const AnimatableValue* to)
     {
         return !from->isSameType(to) || from->usesDefaultInterpolationWith(to);
@@ -132,6 +133,8 @@ private:
     // Implementations can assume that the object being compared has the same type as the object this is called on
     virtual bool equalTo(const AnimatableValue*) const = 0;
 
+    virtual double distanceTo(const AnimatableValue*) const;
+
     friend class KeyframeEffectModel;
 };