Use new downcast helper for blink::SVGSwitchElement

This CL has goal to use IsA<SVGSwitchElement>(element) in place of
IsSVGSwitchElement(element)

Bug: 891908
Change-Id: I82aecebb283a171095b3ff4c58895adeb23c54d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936622
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720920}

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

@@ -47,7 +47,7 @@ bool LayoutSVGTransformableContainer::IsChildAllowed(
     const ComputedStyle& style) const {
   DCHECK(GetElement());
   Node* child_node = child->GetNode();
-  if (IsSVGSwitchElement(*GetElement())) {
+  if (IsA<SVGSwitchElement>(*GetElement())) {
     // Reject non-SVG/non-valid elements.
     auto* svg_element = DynamicTo<SVGElement>(child_node);
     if (!svg_element || !svg_element->IsValid()) {

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

@@ -46,7 +46,7 @@ bool TargetCanHaveMotionTransform(const SVGElement& target) {
   // cause problems.
   return IsA<SVGGElement>(target) || IsA<SVGDefsElement>(target) ||
          IsA<SVGUseElement>(target) || IsA<SVGImageElement>(target) ||
-         IsSVGSwitchElement(target) || IsA<SVGPathElement>(target) ||
+         IsA<SVGSwitchElement>(target) || IsA<SVGPathElement>(target) ||
          IsA<SVGRectElement>(target) || IsA<SVGCircleElement>(target) ||
          IsA<SVGEllipseElement>(target) || IsA<SVGLineElement>(target) ||
          IsA<SVGPolylineElement>(target) || IsA<SVGPolygonElement>(target) ||