Add toSVGFEFuncR|G|B|AElement() fuctions, and use them.

Source/core/svg/SVGFEComponentTransferElement.cpp

Index: Source/core/svg/SVGFEComponentTransferElement.cpp
diff --git a/Source/core/svg/SVGFEComponentTransferElement.cpp b/Source/core/svg/SVGFEComponentTransferElement.cpp
index 96cfa14000338da3b233601e8a17b85f64dfa32f..7b774987736625025420adbe57cdc02141768048 100644
--- a/Source/core/svg/SVGFEComponentTransferElement.cpp
+++ b/Source/core/svg/SVGFEComponentTransferElement.cpp
@@ -90,13 +90,13 @@ PassRefPtr<FilterEffect> SVGFEComponentTransferElement::build(SVGFilterBuilder*
 
     for (Node* node = firstChild(); node; node = node->nextSibling()) {
         if (node->hasTagName(SVGNames::feFuncRTag))
-            red = static_cast<SVGFEFuncRElement*>(node)->transferFunction();
+            red = toSVGFEFuncRElement(node)->transferFunction();
         else if (node->hasTagName(SVGNames::feFuncGTag))
-            green = static_cast<SVGFEFuncGElement*>(node)->transferFunction();
+            green = toSVGFEFuncGElement(node)->transferFunction();
         else if (node->hasTagName(SVGNames::feFuncBTag))
-            blue = static_cast<SVGFEFuncBElement*>(node)->transferFunction();
+            blue = toSVGFEFuncBElement(node)->transferFunction();
         else if (node->hasTagName(SVGNames::feFuncATag))
-            alpha = static_cast<SVGFEFuncAElement*>(node)->transferFunction();
+            alpha = toSVGFEFuncAElement(node)->transferFunction();
     }
 
     RefPtr<FilterEffect> effect = FEComponentTransfer::create(filter, red, green, blue, alpha);