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

Source/core/rendering/RenderListMarker.h

Index: Source/core/rendering/RenderListMarker.h
diff --git a/Source/core/rendering/RenderListMarker.h b/Source/core/rendering/RenderListMarker.h
index 6930949a9875a66f4f904c4006204e9279d6d58c..0f80bc494e6d9b2c417a482ebead0b6e3bd0e7a4 100644
--- a/Source/core/rendering/RenderListMarker.h
+++ b/Source/core/rendering/RenderListMarker.h
@@ -38,6 +38,7 @@ public:
     static RenderListMarker* createAnonymous(RenderListItem*);
 
     virtual ~RenderListMarker();
+    virtual void trace(Visitor*) OVERRIDE;
 
     const String& text() const { return m_text; }
     String suffix() const;
@@ -83,7 +84,7 @@ private:
 
     String m_text;
     RefPtr<StyleImage> m_image;
-    RenderListItem* m_listItem;
+    RawPtrWillBeMember<RenderListItem> m_listItem;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderListMarker, isListMarker());