Oilpan: Make sure XMLTreeViewer::m_document is traceable.

Source/core/xml/XMLTreeViewer.h

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  * 1. Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *
@@ skipping 11 matching lines @@
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef XMLTreeViewer_h
 #define XMLTreeViewer_h
 
+#include "platform/heap/Handle.h"
+
 namespace WebCore {
 
 class Document;
 
 class XMLTreeViewer FINAL {
+    STACK_ALLOCATED();
 public:
     explicit XMLTreeViewer(Document*);
 
     void transformDocumentToTreeView();
 
 private:
-    Document* m_document;
+    RawPtrWillBeMember<Document> m_document;
 };
 
 } // namespace WebCore
 
 #endif // XMLTreeViewer_h