Don't walk treescopes in updateActiveStyleSheets until the resolver exists.

sky/engine/core/dom/StyleSheetCollection.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 21 matching lines @@
 #include "sky/engine/wtf/FastAllocBase.h"
 #include "sky/engine/wtf/PassOwnPtr.h"
 #include "sky/engine/wtf/RefPtr.h"
 #include "sky/engine/wtf/Vector.h"
 
 namespace blink {
 
 class CSSStyleSheet;
 class ContainerNode;
 class HTMLStyleElement;
-class StyleEngine;
+class StyleResolver;
 class TreeScope;
 
 class StyleSheetCollection {
     WTF_MAKE_NONCOPYABLE(StyleSheetCollection);
     WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<StyleSheetCollection> create(TreeScope& treeScope)
     {
         return adoptPtr(new StyleSheetCollection(treeScope));
     }
     ~StyleSheetCollection();
 
     Vector<RefPtr<CSSStyleSheet> >& activeAuthorStyleSheets() { return m_activeAuthorStyleSheets; }
     const Vector<RefPtr<CSSStyleSheet> >& activeAuthorStyleSheets() const { return m_activeAuthorStyleSheets; }
 
     void addStyleSheetCandidateNode(HTMLStyleElement&);
     void removeStyleSheetCandidateNode(HTMLStyleElement&);
 
-    void updateActiveStyleSheets(StyleEngine*);
+    void updateActiveStyleSheets(StyleResolver&);
 
 private:
     explicit StyleSheetCollection(TreeScope&);
 
     void collectStyleSheets(Vector<RefPtr<CSSStyleSheet>>& candidateSheets);
 
     TreeScope& m_treeScope;
     DocumentOrderedList m_styleSheetCandidateNodes;    
     Vector<RefPtr<CSSStyleSheet>> m_activeAuthorStyleSheets;
 
     bool m_needsUpdate;
 };
 
 }
 
 #endif  // SKY_ENGINE_CORE_DOM_STYLESHEETCOLLECTION_H_