Add toSVGFEMergeNodeElement() fuction, and use it.

Source/core/svg/SVGFEMergeElement.cpp

Index: Source/core/svg/SVGFEMergeElement.cpp
diff --git a/Source/core/svg/SVGFEMergeElement.cpp b/Source/core/svg/SVGFEMergeElement.cpp
index caeb9ff756a3a6d7be1e90f4577812cf29c08f91..401fc30a46c75c2fbc09f5db9d87a47eac9d9a0f 100644
--- a/Source/core/svg/SVGFEMergeElement.cpp
+++ b/Source/core/svg/SVGFEMergeElement.cpp
@@ -47,7 +47,7 @@ PassRefPtr<FilterEffect> SVGFEMergeElement::build(SVGFilterBuilder* filterBuilde
     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);