Add toSVGFEMergeNodeElement() fuction, and use it.

Source/core/svg/SVGFEMergeElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2007 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006 Rob Buis <buis@kde.org>
  *
  * 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 29 matching lines @@
 {
     return adoptRef(new SVGFEMergeElement(tagName, document));
 }
 
 PassRefPtr<FilterEffect> SVGFEMergeElement::build(SVGFilterBuilder* filterBuilder, Filter* filter)
 {
     RefPtr<FilterEffect> effect = FEMerge::create(filter);
     FilterEffectVector& mergeInputs = effect->inputEffects();
     for (Node* node = firstChild(); node; node = node->nextSibling()) {
         if (node->hasTagName(SVGNames::feMergeNodeTag)) {
-            FilterEffect* mergeEffect = filterBuilder->getEffectById(static_cast<SVGFEMergeNodeElement*>(node)->in1CurrentValue());
+            FilterEffect* mergeEffect = filterBuilder->getEffectById(toSVGFEMergeNodeElement(node)->in1CurrentValue());
             if (!mergeEffect)
                 return 0;
             mergeInputs.append(mergeEffect);
         }
     }
 
     if (mergeInputs.isEmpty())
         return 0;
 
     return effect.release();
 }
 
 }