Oilpan: fix build after r191432 (5bc98fff).

Source/core/animation/animatable/AnimatableLength.h

Index: Source/core/animation/animatable/AnimatableLength.h
diff --git a/Source/core/animation/animatable/AnimatableLength.h b/Source/core/animation/animatable/AnimatableLength.h
index 7a9c3f10ef4ff48b6666f7c68699362c049b1e56..4c31ca6d5f50ea1955ac84eddef0bb3a94dbc7b7 100644
--- a/Source/core/animation/animatable/AnimatableLength.h
+++ b/Source/core/animation/animatable/AnimatableLength.h
@@ -44,6 +44,8 @@ public:
     }
     Length length(float zoom, ValueRange) const;
 
+    DEFINE_INLINE_VIRTUAL_TRACE() { AnimatableValue::trace(visitor); }
+
 protected:
     virtual PassRefPtrWillBeRawPtr<AnimatableValue> interpolateTo(const AnimatableValue*, double fraction) const override;
 
@@ -64,8 +66,6 @@ private:
     virtual AnimatableType type() const override { return TypeLength; }
     virtual bool equalTo(const AnimatableValue*) const override;
 
-    DEFINE_INLINE_VIRTUAL_TRACE() { AnimatableValue::trace(visitor); }
-
     double m_pixels;
     double m_percent;
     bool m_hasPixels;