Oilpan: fix build after r188198.

Source/core/animation/ImageStyleInterpolation.cpp

 // 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.
 
 #include "config.h"
 #include "core/animation/ImageStyleInterpolation.h"
 
 #include "core/css/resolver/StyleBuilder.h"
 
 
 namespace blink {
 
 bool ImageStyleInterpolation::canCreateFrom(const CSSValue& value)
 {
     return value.isImageValue();
 }
 
 void ImageStyleInterpolation::apply(StyleResolverState& state) const
 {
     double cachedValue = toInterpolableNumber(m_cachedValue.get())->value();
     if (cachedValue <= 0.0) {
         StyleBuilder::applyProperty(m_id, state, m_initialImage.get());
         return;
     }
     if (cachedValue >= 1.0) {
         StyleBuilder::applyProperty(m_id, state, m_finalImage.get());
         return;
     }
-    RefPtr<CSSCrossfadeValue> crossfadeValue = CSSCrossfadeValue::create(m_initialImage, m_finalImage);
+    RefPtrWillBeRawPtr<CSSCrossfadeValue> crossfadeValue = CSSCrossfadeValue::create(m_initialImage, m_finalImage);
     crossfadeValue->setPercentage(CSSPrimitiveValue::create(cachedValue, CSSPrimitiveValue::CSS_NUMBER));
 
     StyleBuilder::applyProperty(m_id, state, crossfadeValue.get());
 }
+
+void ImageStyleInterpolation::trace(Visitor* visitor)
+{
+    visitor->trace(m_initialImage);
+    visitor->trace(m_finalImage);
+    StyleInterpolation::trace(visitor);
 }
+
+} // namespace blink