[Oilpan]: Make StylePropertySet fully garbage collected.

Source/core/animation/interpolation/LengthStyleInterpolationTest.cpp

Index: Source/core/animation/interpolation/LengthStyleInterpolationTest.cpp
diff --git a/Source/core/animation/interpolation/LengthStyleInterpolationTest.cpp b/Source/core/animation/interpolation/LengthStyleInterpolationTest.cpp
index 8120e846fdfee41a984b49e00eaf353316e909bf..82848d433cf5fbed2bd4a1fdba5bc8103df3cafd 100644
--- a/Source/core/animation/interpolation/LengthStyleInterpolationTest.cpp
+++ b/Source/core/animation/interpolation/LengthStyleInterpolationTest.cpp
@@ -63,7 +63,7 @@ protected:
     CSSLengthArray& setLengthArray(CSSLengthArray& lengthArray, String text)
     {
         initLengthArray(lengthArray);
-        RefPtr<MutableStylePropertySet> propertySet = MutableStylePropertySet::create();
+        RefPtrWillBeRawPtr<MutableStylePropertySet> propertySet = MutableStylePropertySet::create();
         propertySet->setProperty(CSSPropertyLeft, text);
         toCSSPrimitiveValue(propertySet->getPropertyCSSValue(CSSPropertyLeft).get())->accumulateLengthArray(lengthArray);
         return lengthArray;