Use new downcast helper for blink::SVGFEFuncAElement

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

Bug: 891908
Change-Id: I65623bbd4c08093fc2f0e8bcdc7d565c79d1adc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936577Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#720254}

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

@@ -75,7 +75,7 @@ FilterEffect* SVGFEComponentTransferElement::Build(
       green = func_g->TransferFunction();
     else if (auto* func_b = ToSVGFEFuncBElementOrNull(*element))
       blue = func_b->TransferFunction();
-    else if (auto* func_a = ToSVGFEFuncAElementOrNull(*element))
+    else if (auto* func_a = DynamicTo<SVGFEFuncAElement>(*element))
       alpha = func_a->TransferFunction();
   }