Oilpan: fix build after r187337.

Source/core/animation/LengthPairStyleInterpolation.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 LengthPairStyleInterpolation_h
 #define LengthPairStyleInterpolation_h
 
 #include "core/animation/StyleInterpolation.h"
 #include "platform/Length.h"
 
 namespace blink {
 
 class LengthPairStyleInterpolation : public StyleInterpolation {
 public:
     static PassRefPtrWillBeRawPtr<LengthPairStyleInterpolation> create(const CSSValue& start, const CSSValue& end, CSSPropertyID id, ValueRange range)
     {
         return adoptRefWillBeNoop(new LengthPairStyleInterpolation(lengthPairToInterpolableValue(start), lengthPairToInterpolableValue(end), id, range));
     }
 
     static bool canCreateFrom(const CSSValue&);
 
     virtual void apply(StyleResolverState&) const override;
-    virtual void trace(Visitor*) override;
 
 private:
     LengthPairStyleInterpolation(PassOwnPtrWillBeRawPtr<InterpolableValue> start, PassOwnPtrWillBeRawPtr<InterpolableValue> end, CSSPropertyID id, ValueRange range)
         : StyleInterpolation(start, end, id), m_range(range)
     {
     }
 
     static PassOwnPtrWillBeRawPtr<InterpolableValue> lengthPairToInterpolableValue(const CSSValue&);
     static PassRefPtrWillBeRawPtr<CSSValue> interpolableValueToLengthPair(InterpolableValue*, ValueRange);
 
     ValueRange m_range;
 
     friend class LengthPairStyleInterpolationTest;
 };
 
 } // namespace blink
 
 #endif // LengthPairStyleInterpolation_h