Remove CSSOM mutability in StyleRule.

sky/engine/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 360 matching lines @@
 
 void StyleEngine::markDocumentDirty()
 {
     m_documentScopeDirty = true;
     if (document().importLoader())
         document().importsController()->master()->styleEngine()->markDocumentDirty();
 }
 
 static bool isCacheableForStyleElement(const StyleSheetContents& contents)
 {
-    // Until import rules are supported in cached sheets it's not possible for loading to fail.
-    ASSERT(!contents.didLoadErrorOccur());
-    // It is not the original sheet anymore.
-    if (contents.isMutable())
-        return false;
     if (!contents.hasSyntacticallyValidCSSHeader())
         return false;
     return true;
 }
 
 PassRefPtr<CSSStyleSheet> StyleEngine::createSheet(Element* e, const String& text, TextPosition startPosition, bool createdByParser)
 {
     RefPtr<CSSStyleSheet> styleSheet = nullptr;
 
     AtomicString textContent(text);
@@ skipping 46 matching lines @@
 {
     if (!document().isActive())
         return;
 
     if (m_resolver)
         m_resolver->invalidateMatchedPropertiesCache();
     document().setNeedsStyleRecalc(SubtreeStyleChange);
 }
 
 }