Oilpan: fix build after r188198.

Source/core/animation/ImageStyleInterpolation.h

 // Copyright 2015 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 ImageStyleInterpolation_h
 #define ImageStyleInterpolation_h
 
 #include "core/animation/StyleInterpolation.h"
 #include "core/css/CSSCrossfadeValue.h"
 
 namespace blink {
 class ImageStyleInterpolation : public StyleInterpolation {
 public:
     static PassRefPtrWillBeRawPtr<ImageStyleInterpolation> create(CSSValue& start, CSSValue& end, CSSPropertyID id)
     {
         return adoptRefWillBeNoop(new ImageStyleInterpolation(&start, &end, id));
     }
 
+    virtual void trace(Visitor*) override;
+
     static bool canCreateFrom(const CSSValue&);
     virtual void apply(StyleResolverState&) const override;
 
 private:
     ImageStyleInterpolation(PassRefPtrWillBeRawPtr<CSSValue> start, PassRefPtrWillBeRawPtr<CSSValue> end, CSSPropertyID id)
         : StyleInterpolation(InterpolableNumber::create(0.0), InterpolableNumber::create(1.0), id)
         , m_initialImage(start)
         , m_finalImage(end)
     {
     }
 
-    PassRefPtrWillBeRawPtr<CSSValue> m_initialImage;
-    PassRefPtrWillBeRawPtr<CSSValue> m_finalImage;
+    RefPtrWillBeMember<CSSValue> m_initialImage;
+    RefPtrWillBeMember<CSSValue> m_finalImage;
 };
 }
 
 #endif // ImageStyleInterpolation_h