Use type cast macro for AnimatableLengthPoint|3D instead of manual toFoo()

Source/core/animation/interpolation/LegacyStyleInterpolation.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 LegacyStyleInterpolation_h
 #define LegacyStyleInterpolation_h
 
 #include "core/animation/interpolation/StyleInterpolation.h"
 #include "core/css/resolver/AnimatedStyleBuilder.h"
 
 namespace WebCore {
 
 class LegacyStyleInterpolation : public StyleInterpolation {
 public:
     static PassRefPtrWillBeRawPtr<LegacyStyleInterpolation> create(PassRefPtrWillBeRawPtr<AnimatableValue> start, PassRefPtrWillBeRawPtr<AnimatableValue> end, CSSPropertyID id)
     {
         return adoptRefWillBeNoop(new LegacyStyleInterpolation(InterpolableAnimatableValue::create(start), InterpolableAnimatableValue::create(end), id));
     }
 
     virtual void apply(StyleResolverState& state) const OVERRIDE
     {
         AnimatedStyleBuilder::applyProperty(m_id, state, currentValue().get());
     }
 
     virtual bool isLegacyStyleInterpolation() const OVERRIDE FINAL { return true; }
     PassRefPtrWillBeRawPtr<AnimatableValue> currentValue() const
     {
-        InterpolableAnimatableValue* value = static_cast<InterpolableAnimatableValue*>(m_cachedValue.get());
-        return value->value();
+        return toInterpolableAnimatableValue(m_cachedValue.get())->value();
     }
 
     virtual void trace(Visitor* visitor) OVERRIDE
     {
         StyleInterpolation::trace(visitor);
     }
 
 private:
     LegacyStyleInterpolation(PassOwnPtrWillBeRawPtr<InterpolableValue> start, PassOwnPtrWillBeRawPtr<InterpolableValue> end, CSSPropertyID id)
         : StyleInterpolation(start, end, id)
     {
     }
 };
 
 DEFINE_TYPE_CASTS(LegacyStyleInterpolation, Interpolation, value, value->isLegacyStyleInterpolation(), value.isLegacyStyleInterpolation());
 
 }
 
 #endif