Move StyleSheet to the oilpan heap using transition types.

Source/core/css/CSSRule.h

Index: Source/core/css/CSSRule.h
diff --git a/Source/core/css/CSSRule.h b/Source/core/css/CSSRule.h
index fb5f69242c3dcd012046385b05d0d19cc8d2104f..519ed90a86cf8a39fd58a026e6fed88daf164be2 100644
--- a/Source/core/css/CSSRule.h
+++ b/Source/core/css/CSSRule.h
@@ -108,8 +108,9 @@ private:
     mutable unsigned char m_hasCachedSelectorText : 1;
     unsigned char m_parentIsRule : 1;
 
+    // These should be Members, but no Members in unions.
     union {
-        CSSRule* m_parentRule; // Should be Member, but no Members in unions.
+        CSSRule* m_parentRule;
         CSSStyleSheet* m_parentStyleSheet;
     };
 };