Make sure we never pass a null Frame to Document::canNavigate()

Source/core/dom/Document.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 561 matching lines @@
     const KURL& baseElementURL() const { return m_baseElementURL; }
     const AtomicString& baseTarget() const { return m_baseTarget; }
     void processBaseElement();
 
     KURL completeURL(const String&) const;
     KURL completeURLWithOverride(const String&, const KURL& baseURLOverride) const;
 
     virtual String userAgent(const KURL&) const OVERRIDE FINAL;
     virtual void disableEval(const String& errorMessage) OVERRIDE FINAL;
 
-    bool canNavigate(Frame* targetFrame);
+    bool canNavigate(const Frame& targetFrame);
     LocalFrame* findUnsafeParentScrollPropagationBoundary();
 
     CSSStyleSheet& elementSheet();
 
     virtual PassRefPtrWillBeRawPtr<DocumentParser> createParser();
     DocumentParser* parser() const { return m_parser.get(); }
     ScriptableDocumentParser* scriptableDocumentParser() const;
 
     bool printing() const { return m_printing; }
     void setPrinting(bool p) { m_printing = p; }
@@ skipping 851 matching lines @@
 inline bool Node::isDocumentNode() const
 {
     return this == document();
 }
 
 Node* eventTargetNodeForDocument(Document*);
 
 } // namespace WebCore
 
 #endif // Document_h