Oilpan: Build fix after r178893

Source/core/animation/css/CSSAnimatableValueFactory.cpp

Index: Source/core/animation/css/CSSAnimatableValueFactory.cpp
diff --git a/Source/core/animation/css/CSSAnimatableValueFactory.cpp b/Source/core/animation/css/CSSAnimatableValueFactory.cpp
index f52b2f61ee07b6f19d81207feefb96f9fa08acd0..c32c2927dbf3be8fcd496f17198a23945fa3d26e 100644
--- a/Source/core/animation/css/CSSAnimatableValueFactory.cpp
+++ b/Source/core/animation/css/CSSAnimatableValueFactory.cpp
@@ -230,7 +230,7 @@ static double fontStretchToDouble(FontStretch fontStretch)
     return static_cast<unsigned>(fontStretch);
 }
 
-static PassRefPtr<AnimatableValue> createFromFontStretch(FontStretch fontStretch)
+static PassRefPtrWillBeRawPtr<AnimatableValue> createFromFontStretch(FontStretch fontStretch)
 {
     return createFromDouble(fontStretchToDouble(fontStretch));
 }