Check if stylesheet resource was cached before marking it as such.

third_party/WebKit/Source/core/css/StyleSheetContents.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 561 matching lines @@
 
 void StyleSheetContents::addedToMemoryCache()
 {
     ASSERT(!m_isInMemoryCache);
     ASSERT(isCacheable());
     m_isInMemoryCache = true;
 }
 
 void StyleSheetContents::removedFromMemoryCache()
 {
-    ASSERT(m_isInMemoryCache);
     ASSERT(isCacheable());
     m_isInMemoryCache = false;
 }
 
 RuleSet& StyleSheetContents::ensureRuleSet(const MediaQueryEvaluator& medium, AddRuleFlags addRuleFlags)
 {
     if (!m_ruleSet) {
         m_ruleSet = RuleSet::create();
         m_ruleSet->addRulesFromSheet(this, medium, addRuleFlags);
     }
@@ skipping 75 matching lines @@
     visitor->trace(m_importRules);
     visitor->trace(m_namespaceRules);
     visitor->trace(m_childRules);
     visitor->trace(m_loadingClients);
     visitor->trace(m_completedClients);
     visitor->trace(m_ruleSet);
 #endif
 }
 
 } // namespace blink