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

Source/core/css/CSSGrammar.y

Index: Source/core/css/CSSGrammar.y
diff --git a/Source/core/css/CSSGrammar.y b/Source/core/css/CSSGrammar.y
index 65c24613abe1b07027e56cbd26936cae5ba40c44..b1e315fc4589cea52ee80f8343ce28301e08f7e8 100644
--- a/Source/core/css/CSSGrammar.y
+++ b/Source/core/css/CSSGrammar.y
@@ -73,8 +73,10 @@ using namespace HTMLNames;
     CSSParserString string;
 
     StyleRuleBase* rule;
-    // Vector pointed to by raw ruleList pointer is guaranteed to be kept alive by the BisonCSSParser::m_parsedRules vector.
+    // The content of the two below HeapVectors are guaranteed to be kept alive by
+    // the corresponding m_parsedRules and m_floatingMediaQueryExpList lists in BisonCSSParser.h.
     WillBeHeapVector<RefPtrWillBeMember<StyleRuleBase> >* ruleList;
+    WillBeHeapVector<OwnPtrWillBeMember<MediaQueryExp> >* mediaQueryExpList;
     CSSParserSelector* selector;
     Vector<OwnPtr<CSSParserSelector> >* selectorList;
     CSSSelector::MarginBoxType marginBox;
@@ -85,7 +87,6 @@ using namespace HTMLNames;
     MediaQueryExp* mediaQueryExp;
     CSSParserValue value;
     CSSParserValueList* valueList;
-    Vector<OwnPtr<MediaQueryExp> >* mediaQueryExpList;
     StyleKeyframe* keyframe;
     Vector<RefPtr<StyleKeyframe> >* keyframeRuleList;
     float val;