Oilpan: fix build after r188198.

Source/core/animation/ImageStyleInterpolation.h

Index: Source/core/animation/ImageStyleInterpolation.h
diff --git a/Source/core/animation/ImageStyleInterpolation.h b/Source/core/animation/ImageStyleInterpolation.h
index 30d5400e9457ad474de401971838e9be5e208ba2..35395a279cdb293d367f425edc587d34b2c2c134 100644
--- a/Source/core/animation/ImageStyleInterpolation.h
+++ b/Source/core/animation/ImageStyleInterpolation.h
@@ -16,6 +16,8 @@ public:
         return adoptRefWillBeNoop(new ImageStyleInterpolation(&start, &end, id));
     }
 
+    virtual void trace(Visitor*) override;
+
     static bool canCreateFrom(const CSSValue&);
     virtual void apply(StyleResolverState&) const override;
 
@@ -27,8 +29,8 @@ private:
     {
     }
 
-    PassRefPtrWillBeRawPtr<CSSValue> m_initialImage;
-    PassRefPtrWillBeRawPtr<CSSValue> m_finalImage;
+    RefPtrWillBeMember<CSSValue> m_initialImage;
+    RefPtrWillBeMember<CSSValue> m_finalImage;
 };
 }