Use new downcast helper for blink::SVGFEDropShadowElement

This CL has goal to use IsA<SVGFEDropShadowElement>(element) in place of
IsSVGFEDropShadowElement(element)

Bug: 891908
Change-Id: I01b8c852bb487493847267af2dd5ea7ffeede0d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936575
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720453}

third_party/blink/renderer/core/layout/svg/layout_svg_resource_filter_primitive.cc

@@ -61,7 +61,7 @@ void LayoutSVGResourceFilterPrimitive::StyleDidChange(
   DCHECK(GetElement());
   auto& element = To<SVGFilterPrimitiveStandardAttributes>(*GetElement());
   const SVGComputedStyle& new_style = StyleRef().SvgStyle();
-  if (IsA<SVGFEFloodElement>(element) || IsSVGFEDropShadowElement(element)) {
+  if (IsA<SVGFEFloodElement>(element) || IsA<SVGFEDropShadowElement>(element)) {
     CheckForColorChange(element, svg_names::kFloodColorAttr, diff,
                         old_style->SvgStyle().FloodColor(),
                         new_style.FloodColor());