Use new downcast helper for blink::SVGFEFuncBElement

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

Bug: 891908
Change-Id: I8a374d96a16c68707a06ae96f1cb1f1353213b03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936578Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#720285}

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

@@ -73,7 +73,7 @@ FilterEffect* SVGFEComponentTransferElement::Build(
       red = func_r->TransferFunction();
     else if (auto* func_g = DynamicTo<SVGFEFuncGElement>(*element))
       green = func_g->TransferFunction();
-    else if (auto* func_b = ToSVGFEFuncBElementOrNull(*element))
+    else if (auto* func_b = DynamicTo<SVGFEFuncBElement>(*element))
       blue = func_b->TransferFunction();
     else if (auto* func_a = DynamicTo<SVGFEFuncAElement>(*element))
       alpha = func_a->TransferFunction();