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

sky/engine/core/dom/StyleEngine.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 41 matching lines @@
 public:
     static PassOwnPtr<StyleEngine> create(Document& document) { return adoptPtr(new StyleEngine(document)); }
 
     ~StyleEngine();
 
     void detachFromDocument();
 
     void addTreeScope(TreeScope&);
     void removeTreeScope(TreeScope&);
 
-    void updateActiveStyleSheets();
-
     // FIXME(sky): Remove this and ::first-line.
     bool usesFirstLineRules() const { return false; }
 
     void appendActiveAuthorStyleSheets();
 
     StyleResolver* resolver() const
     {
         return m_resolver.get();
     }
 
@@ skipping 21 matching lines @@
     PassRefPtr<CSSStyleSheet> createSheet(Element*, const String& text);
     void removeSheet(StyleSheetContents*);
 
 private:
     // CSSFontSelectorClient implementation.
     virtual void fontsNeedUpdate(CSSFontSelector*) override;
 
 private:
     explicit StyleEngine(Document&);
 
+    void updateActiveStyleSheets();
     void createResolver();
 
     RawPtr<Document> m_document;
 
     typedef ListHashSet<TreeScope*, 16> TreeScopeSet;
     TreeScopeSet m_activeTreeScopes;
 
     OwnPtr<StyleResolver> m_resolver;
 
     RefPtr<CSSFontSelector> m_fontSelector;
 
     HashMap<AtomicString, StyleSheetContents*> m_textToSheetCache;
     HashMap<StyleSheetContents*, AtomicString> m_sheetToTextCache;
 };
 
 }
 
 #endif  // SKY_ENGINE_CORE_DOM_STYLEENGINE_H_