Change various helper classes to transition types to get CSSValue entirely onto the gc heap.

Source/core/css/CSSImportRule.h

Index: Source/core/css/CSSImportRule.h
diff --git a/Source/core/css/CSSImportRule.h b/Source/core/css/CSSImportRule.h
index 95df9c7c1f986dfead3d6a1472ce943d818f8408..f26fa3e37155e65477cdcbdd80781ca858c78462 100644
--- a/Source/core/css/CSSImportRule.h
+++ b/Source/core/css/CSSImportRule.h
@@ -23,6 +23,7 @@
 #define CSSImportRule_h
 
 #include "core/css/CSSRule.h"
+#include "heap/Handle.h"
 
 namespace WebCore {
 
@@ -49,7 +50,7 @@ private:
     CSSImportRule(StyleRuleImport*, CSSStyleSheet*);
 
     RefPtr<StyleRuleImport> m_importRule;
-    mutable RefPtr<MediaList> m_mediaCSSOMWrapper;
+    mutable RefPtrWillBePersistent<MediaList> m_mediaCSSOMWrapper;
     mutable RefPtr<CSSStyleSheet> m_styleSheetCSSOMWrapper;
 };