Use isSVG*Element() helpers more in SVG code (Part 3)

Source/core/rendering/svg/RenderSVGResourceMasker.cpp

Index: Source/core/rendering/svg/RenderSVGResourceMasker.cpp
diff --git a/Source/core/rendering/svg/RenderSVGResourceMasker.cpp b/Source/core/rendering/svg/RenderSVGResourceMasker.cpp
index 82f8a30226428d363584f5edacb54941f9e66b39..6bcf50dc97af5157758de893f411b91062abe3ff 100644
--- a/Source/core/rendering/svg/RenderSVGResourceMasker.cpp
+++ b/Source/core/rendering/svg/RenderSVGResourceMasker.cpp
@@ -153,9 +153,9 @@ PassRefPtr<DisplayList> RenderSVGResourceMasker::asDisplayList(GraphicsContext*
 
 void RenderSVGResourceMasker::calculateMaskContentRepaintRect()
 {
-    for (Element* childElement = ElementTraversal::firstWithin(*element()); childElement; childElement = ElementTraversal::nextSibling(*childElement)) {
+    for (SVGElement* childElement = Traversal<SVGElement>::firstChild(*element()); childElement; childElement = Traversal<SVGElement>::nextSibling(*childElement)) {
         RenderObject* renderer = childElement->renderer();
-        if (!childElement->isSVGElement() || !renderer)
+        if (!renderer)
             continue;
         RenderStyle* style = renderer->style();
         if (!style || style->display() == NONE || style->visibility() != VISIBLE)