Oilpan: fix webkit unit tests after r191696 (d325d00a5).

Source/core/animation/LengthStyleInterpolation.cpp

Index: Source/core/animation/LengthStyleInterpolation.cpp
diff --git a/Source/core/animation/LengthStyleInterpolation.cpp b/Source/core/animation/LengthStyleInterpolation.cpp
index 81f9fdf05e24bc812fa8e364a5fd22948c23217d..70ddf74610c389f2e05696b38168e5d5e3c96a0a 100644
--- a/Source/core/animation/LengthStyleInterpolation.cpp
+++ b/Source/core/animation/LengthStyleInterpolation.cpp
@@ -207,7 +207,7 @@ static PassRefPtrWillBeRawPtr<CSSCalcExpressionNode> constructCalcExpression(con
 {
     const InterpolableList* listOfValues = toInterpolableList(list->get(0));
     const InterpolableList* listOfTypes = toInterpolableList(list->get(1));
-    RefPtrWillBeRawPtr<CSSCalcExpressionNode> expression;
+    RefPtrWillBeRawPtr<CSSCalcExpressionNode> expression = nullptr;
     for (size_t position = 0; position < CSSPrimitiveValue::LengthUnitTypeCount; position++) {
         const InterpolableNumber *subValueType = toInterpolableNumber(listOfTypes->get(position));
         if (!subValueType->value())