Have Document::topDocument() return a reference

Source/core/accessibility/AXObjectCache.h

Index: Source/core/accessibility/AXObjectCache.h
diff --git a/Source/core/accessibility/AXObjectCache.h b/Source/core/accessibility/AXObjectCache.h
index 6d8270a1470a054b01be73f45b5608fdd75f2812..5024525183b484ce9210903c7836a58f9685482f 100644
--- a/Source/core/accessibility/AXObjectCache.h
+++ b/Source/core/accessibility/AXObjectCache.h
@@ -79,7 +79,7 @@ enum PostType { PostSynchronously, PostAsynchronously };
 class AXObjectCache {
     WTF_MAKE_NONCOPYABLE(AXObjectCache); WTF_MAKE_FAST_ALLOCATED;
 public:
-    explicit AXObjectCache(const Document*);
+    explicit AXObjectCache(Document&);
     ~AXObjectCache();
 
     static AXObject* focusedUIElementForPage(const Page*);
@@ -211,7 +211,7 @@ protected:
     bool isNodeInUse(Node* n) { return m_textMarkerNodes.contains(n); }
 
 private:
-    Document* m_document;
+    Document& m_document;
     HashMap<AXID, RefPtr<AXObject> > m_objects;
     HashMap<RenderObject*, AXID> m_renderObjectMapping;
     HashMap<Widget*, AXID> m_widgetObjectMapping;