Null check collection handling dirty scopes.

Source/core/dom/StyleEngine.cpp

 /*
  * 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, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 225 matching lines @@
 void StyleEngine::addPendingSheet()
 {
     m_pendingStylesheets++;
 }
 
 // This method is called whenever a top-level stylesheet has finished loading.
 void StyleEngine::removePendingSheet(Node* styleSheetCandidateNode)
 {
     ASSERT(styleSheetCandidateNode);
     TreeScope* treeScope = isStyleElement(*styleSheetCandidateNode) ? &styleSheetCandidateNode->treeScope() : m_document.get();
-    markTreeScopeDirty(*treeScope);
+    if (styleSheetCandidateNode->inDocument())
+        markTreeScopeDirty(*treeScope);
 
     // Make sure we knew this sheet was pending, and that our count isn't out of sync.
     ASSERT(m_pendingStylesheets > 0);
 
     m_pendingStylesheets--;
     if (m_pendingStylesheets)
         return;
 
     // FIXME: We can't call addedStyleSheet or removedStyleSheet here because we don't know
     // what's new. We should track that to tell the style system what changed.
@@ skipping 315 matching lines @@
         m_resolver->invalidateMatchedPropertiesCache();
 }
 
 void StyleEngine::markTreeScopeDirty(TreeScope& scope)
 {
     if (scope == m_document) {
         markDocumentDirty();
         return;
     }
 
+    ASSERT(m_styleSheetCollectionMap.contains(&scope));
     m_dirtyTreeScopes.add(&scope);
 }
 
 void StyleEngine::markDocumentDirty()
 {
     m_documentScopeDirty = true;
     if (document().importLoader())
         document().importsController()->master()->styleEngine().markDocumentDirty();
 }
 
@@ skipping 113 matching lines @@
     visitor->trace(m_dirtyTreeScopes);
     visitor->trace(m_activeTreeScopes);
     visitor->trace(m_fontSelector);
     visitor->trace(m_textToSheetCache);
     visitor->trace(m_sheetToTextCache);
 #endif
     CSSFontSelectorClient::trace(visitor);
 }
 
 }