Oilpan: Prepare to move RenderObject and RenderObjectChildList to Oilpan heap.

Source/core/rendering/RenderListItem.h

Index: Source/core/rendering/RenderListItem.h
diff --git a/Source/core/rendering/RenderListItem.h b/Source/core/rendering/RenderListItem.h
index a2c32f30c79a12c953af126d33a5a3c33f3263ed..da063bf891bb705fa599f99ddca30649884192e0 100644
--- a/Source/core/rendering/RenderListItem.h
+++ b/Source/core/rendering/RenderListItem.h
@@ -33,6 +33,7 @@ class RenderListMarker;
 class RenderListItem FINAL : public RenderBlockFlow {
 public:
     explicit RenderListItem(Element*);
+    virtual void trace(Visitor*) OVERRIDE;
 
     int value() const { if (!m_isValueUpToDate) updateValueNow(); return m_value; }
     void updateValue();
@@ -83,7 +84,7 @@ private:
     void explicitValueChanged();
 
     int m_explicitValue;
-    RenderListMarker* m_marker;
+    RawPtrWillBeMember<RenderListMarker> m_marker;
     mutable int m_value;
 
     bool m_hasExplicitValue : 1;