[oilpan] The Node hierarchy should have correct accept method chains

Source/core/html/HTMLUnknownElement.h

Index: Source/core/html/HTMLUnknownElement.h
diff --git a/Source/core/html/HTMLUnknownElement.h b/Source/core/html/HTMLUnknownElement.h
index faf3f4e52ca99b7c0f7569cc77a8ef7a7f095d61..cd79fd4ae8714f72ebcc0af079448100e2253b55 100644
--- a/Source/core/html/HTMLUnknownElement.h
+++ b/Source/core/html/HTMLUnknownElement.h
@@ -43,6 +43,11 @@ public:
 
     virtual bool isHTMLUnknownElement() const OVERRIDE { return true; }
 
+    virtual void acceptHeapVisitor(Visitor* visitor) const OVERRIDE
+    {
+        HTMLElement::acceptHeapVisitor(visitor);
+    }
+
 private:
     HTMLUnknownElement(const QualifiedName& tagName, Handle<Document> document)
         : HTMLElement(tagName, document)