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 aefc40a8cdef86bc330b54fbd0059fd7cf1706f2..7fbfd697bde2b4895f59a6dcac32b9cb5f00158d 100644
--- a/Source/core/css/CSSImportRule.h
+++ b/Source/core/css/CSSImportRule.h
@@ -49,13 +49,13 @@ public:
     MediaList* media() const;
     CSSStyleSheet* styleSheet() const;
 
-    virtual void trace(Visitor* visitor) OVERRIDE { CSSRule::trace(visitor); }
+    virtual void trace(Visitor*) OVERRIDE;
 
 private:
     CSSImportRule(StyleRuleImport*, CSSStyleSheet*);
 
-    RefPtrWillBePersistent<StyleRuleImport> m_importRule;
-    mutable RefPtr<MediaList> m_mediaCSSOMWrapper;
+    RefPtrWillBeMember<StyleRuleImport> m_importRule;
+    mutable RefPtrWillBeMember<MediaList> m_mediaCSSOMWrapper;
     mutable RefPtr<CSSStyleSheet> m_styleSheetCSSOMWrapper;
 };