Oilpan: fix build after r198129.

Source/core/inspector/InspectorStyleSheet.cpp

Index: Source/core/inspector/InspectorStyleSheet.cpp
diff --git a/Source/core/inspector/InspectorStyleSheet.cpp b/Source/core/inspector/InspectorStyleSheet.cpp
index 4a5936b1bd7b5c61f73212b69e479a1dd5d7cfb3..3aadde08e61707d2a903477123021ea75935e0b4 100644
--- a/Source/core/inspector/InspectorStyleSheet.cpp
+++ b/Source/core/inspector/InspectorStyleSheet.cpp
@@ -603,7 +603,7 @@ String canonicalCSSText(RefPtrWillBeRawPtr<CSSRule> rule)
 {
     if (rule->type() != CSSRule::STYLE_RULE)
         return rule->cssText();
-    RefPtr<CSSStyleRule> styleRule = toCSSStyleRule(rule.get());
+    RefPtrWillBeRawPtr<CSSStyleRule> styleRule = toCSSStyleRule(rule.get());
 
     Vector<String> propertyNames;
     HashSet<String> propertyNameSet;