Should remove StyleSheetContents from cache even if document is not active.

Source/core/css/CSSStyleSheet.cpp

 /*
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2006, 2007, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 144 matching lines @@
 #endif
 }
 
 void CSSStyleSheet::willMutateRules()
 {
     InspectorInstrumentation::willMutateRules(this);
 
     // If we are the only client it is safe to mutate.
     if (m_contents->clientSize() <= 1 && !m_contents->isInMemoryCache()) {
         m_contents->clearRuleSet();
-        m_contents->removeSheetFromCache(ownerDocument());
+        if (Document* document = ownerDocument())
+            m_contents->removeSheetFromCache(document);
         m_contents->setMutable();
         return;
     }
     // Only cacheable stylesheets should have multiple clients.
     ASSERT(m_contents->isCacheable());
 
     // Copy-on-write.
     m_contents->unregisterClient(this);
     m_contents = m_contents->copy();
     m_contents->registerClient(this);
@@ skipping 275 matching lines @@
 {
     visitor->trace(m_contents);
     visitor->trace(m_mediaQueries);
     visitor->trace(m_ownerRule);
     visitor->trace(m_mediaCSSOMWrapper);
     visitor->trace(m_childRuleCSSOMWrappers);
     visitor->trace(m_ruleListCSSOMWrapper);
 }
 
 }