Consistently use on-heap collections for StyleRuleBase descendants.

Source/core/css/ElementRuleCollector.h

Index: Source/core/css/ElementRuleCollector.h
diff --git a/Source/core/css/ElementRuleCollector.h b/Source/core/css/ElementRuleCollector.h
index dbab0feef00be3b72494b140e7f6b72d588ab131..42dac8d81436362d2937fc6d4a0a3a2e93b765e9 100644
--- a/Source/core/css/ElementRuleCollector.h
+++ b/Source/core/css/ElementRuleCollector.h
@@ -75,10 +75,11 @@ private:
     const CSSStyleSheet* m_parentStyleSheet;
 };
 
+// FIXME: when transition types are gone this class can be replaced with HeapVector.
 class StyleRuleList : public RefCounted<StyleRuleList> {
 public:
     static PassRefPtr<StyleRuleList> create() { return adoptRef(new StyleRuleList()); }
-    Vector<StyleRule*> m_list;
+    WillBePersistentHeapVector<StyleRule*> m_list;
 };
 
 // ElementRuleCollector is designed to be used as a stack object.