Oilpan: Prepare to move AXObjectCache to the heap

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index ea7f64e3b762cbcff13cb65f18ba956eacbf15b6..2173c0c013e836d3a4982e42d7eb845ea48a0c52 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -1104,8 +1104,6 @@ private:
 
     void detachParser();
 
-    void clearWeakMembers(Visitor*);
-
     virtual bool isDocument() const override final { return true; }
 
     virtual void childrenChanged(const ChildrenChange&) override;
@@ -1263,7 +1261,7 @@ private:
     String m_rawTitle;
     RefPtrWillBeMember<Element> m_titleElement;
 
-    OwnPtr<AXObjectCache> m_axObjectCache;
+    OwnPtrWillBeMember<AXObjectCache> m_axObjectCache;
     OwnPtrWillBeMember<DocumentMarkerController> m_markers;
 
     Timer<Document> m_updateFocusAppearanceTimer;