Oilpan: Trace DocumentVisibilityObserver::m_document.

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 3bb563310a5060d65c17d754a97c7798218e964d..824907a7cf40870981f12ecf1af2e2cf2c1a8fe9 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -225,10 +225,14 @@ public:
     // DocumentVisibilityObserver::setDocument
     void setObservedDocument(Document&);
 
+protected:
+    void trace(Visitor*);
+
 private:
     void registerObserver(Document&);
     void unregisterObserver();
-    Document* m_document;
+
+    RawPtrWillBeMember<Document> m_document;
 };
 
 class Document : public ContainerNode, public TreeScope, public SecurityContext, public ExecutionContext, public ExecutionContextClient