Add foundation for removing AnimatableValues from StyleInterpolation

Source/core/animation/InterpolableValue.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef InterpolableValue_h
 #define InterpolableValue_h
 
 #include "core/CoreExport.h"
 #include "core/animation/animatable/AnimatableValue.h"
 #include "platform/heap/Handle.h"
@@ skipping 14 matching lines @@
     virtual PassOwnPtrWillBeRawPtr<InterpolableValue> clone() const = 0;
 
     DEFINE_INLINE_VIRTUAL_TRACE() { }
 
 private:
     virtual void interpolate(const InterpolableValue& to, const double progress, InterpolableValue& result) const = 0;
     virtual void add(const InterpolableValue& rhs, InterpolableValue& result) const = 0;
     virtual void multiply(double scalar, InterpolableValue& result) const = 0;
 
     friend class Interpolation;
+    friend class PairwisePrimitiveInterpolation;
 
     // Keep interpolate private, but allow calls within the hierarchy without
     // knowledge of type.
     friend class DeferredLegacyStyleInterpolation;
     friend class InterpolableNumber;
     friend class InterpolableBool;
     friend class InterpolableList;
 
     friend class AnimationInterpolableValueTest;
 };
@@ skipping 131 matching lines @@
 };
 
 DEFINE_TYPE_CASTS(InterpolableNumber, InterpolableValue, value, value->isNumber(), value.isNumber());
 DEFINE_TYPE_CASTS(InterpolableBool, InterpolableValue, value, value->isBool(), value.isBool());
 DEFINE_TYPE_CASTS(InterpolableList, InterpolableValue, value, value->isList(), value.isList());
 DEFINE_TYPE_CASTS(InterpolableAnimatableValue, InterpolableValue, value, value->isAnimatableValue(), value.isAnimatableValue());
 
 }
 
 #endif