Use new downcast helper for blink::SVGFEFuncRElement

This CL has goal to use DynamicTo<SVGFEFuncRElement> as new downcast
helper

Bug: 891908
Change-Id: Iff93cc32f1f3052db517a13bd93d621448cedf80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936581Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#720314}

third_party/blink/renderer/core/svg/svg_fe_component_transfer_element.cc

@@ -69,7 +69,7 @@ FilterEffect* SVGFEComponentTransferElement::Build(
 
   for (SVGElement* element = Traversal<SVGElement>::FirstChild(*this); element;
        element = Traversal<SVGElement>::NextSibling(*element)) {
-    if (auto* func_r = ToSVGFEFuncRElementOrNull(*element))
+    if (auto* func_r = DynamicTo<SVGFEFuncRElement>(*element))
       red = func_r->TransferFunction();
     else if (auto* func_g = DynamicTo<SVGFEFuncGElement>(*element))
       green = func_g->TransferFunction();