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

Source/core/css/CSSImportRule.cpp

Index: Source/core/css/CSSImportRule.cpp
diff --git a/Source/core/css/CSSImportRule.cpp b/Source/core/css/CSSImportRule.cpp
index cf733f70f3c4bf913a9d2bc5fafec498ea05f083..9525f27ae4cd5a6c341a81d01418a16d529ce7e7 100644
--- a/Source/core/css/CSSImportRule.cpp
+++ b/Source/core/css/CSSImportRule.cpp
@@ -40,8 +40,12 @@ CSSImportRule::~CSSImportRule()
 {
     if (m_styleSheetCSSOMWrapper)
         m_styleSheetCSSOMWrapper->clearOwnerRule();
+#if !ENABLE(OILPAN)
+    // Oilpan automatically handles cycles and doesn't want destructors to touch potentially

[Mads Ager (chromium), 2014/03/03 10:38:45]

Maybe the comment should state that the MediaList and its parent CSSImport rules
died together with oilpan and therefore the clearing is not necessary?

[wibling-chromium, 2014/03/03 10:45:08]

Done.

+    // dead objects to perform cleanup.
     if (m_mediaCSSOMWrapper)
         m_mediaCSSOMWrapper->clearParentRule();
+#endif // ENABLE(OILPAN)
 }
 
 String CSSImportRule::href() const
@@ -91,4 +95,11 @@ void CSSImportRule::reattach(StyleRuleBase*)
     ASSERT_NOT_REACHED();
 }
 
+void CSSImportRule::trace(Visitor* visitor)
+{
+    visitor->trace(m_importRule);
+    visitor->trace(m_mediaCSSOMWrapper);
+    CSSRule::trace(visitor);
+}
+
 } // namespace WebCore