Introduce CSS_VALUE_TYPE_CASTS macro to cast CSSValue

Source/core/css/resolver/ElementStyleResources.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google 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.
  *
@@ skipping 32 matching lines @@
     if (value->isImageValue())
         return cachedOrPendingFromValue(property, toCSSImageValue(value));
 
     if (value->isImageGeneratorValue()) {
         if (value->isGradientValue())
             return generatedOrPendingFromValue(property, static_cast<CSSGradientValue*>(value)->gradientWithStylesResolved(textLinkColors, currentColor).get());
         return generatedOrPendingFromValue(property, static_cast<CSSImageGeneratorValue*>(value));
     }
 
     if (value->isImageSetValue())
-        return setOrPendingFromValue(property, static_cast<CSSImageSetValue*>(value));
+        return setOrPendingFromValue(property, toCSSImageSetValue(value));
 
     if (value->isCursorImageValue())
         return cursorOrPendingFromValue(property, static_cast<CSSCursorImageValue*>(value));
 
     return 0;
 }
 
 PassRefPtr<StyleImage> ElementStyleResources::generatedOrPendingFromValue(CSSPropertyID property, CSSImageGeneratorValue* value)
 {
     if (value->isPending()) {
@@ skipping 34 matching lines @@
 
 void ElementStyleResources::clear()
 {
     m_pendingImageProperties.clear();
     m_pendingSVGDocuments.clear();
     m_hasPendingShaders = false;
     m_deviceScaleFactor = 1;
 }
 
 }