Oilpan: Make sure XMLTreeViewer::m_document is traceable.

- Mark XMLTreeViewer STACK_ALLOCATED().
- Make m_document Member<>.

BUG=357163

Review URL: https://codereview.chromium.org/339703004

git-svn-id: svn://svn.chromium.org/blink/trunk@176263 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/xml/XMLTreeViewer.h

@@ -29,18 +29,21 @@
 #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