Use new downcast helper for blink::SVGPolygonElement

This CL has goal to use IsA<SVGPolygonElement>(element) in place of
IsSVGPolygonElement(element)

Bug: 891908
Change-Id: I7a04d9cf58093e1b244670d1e4dc6466849b60b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936615
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720133}

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

@@ -49,7 +49,7 @@ bool TargetCanHaveMotionTransform(const SVGElement& target) {
          IsSVGSwitchElement(target) || IsSVGPathElement(target) ||
          IsSVGRectElement(target) || IsSVGCircleElement(target) ||
          IsSVGEllipseElement(target) || IsA<SVGLineElement>(target) ||
-         IsA<SVGPolylineElement>(target) || IsSVGPolygonElement(target) ||
+         IsA<SVGPolylineElement>(target) || IsA<SVGPolygonElement>(target) ||
          IsSVGTextElement(target) || IsSVGClipPathElement(target) ||
          IsSVGMaskElement(target) || IsSVGAElement(target) ||
          IsSVGForeignObjectElement(target);

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

@@ -499,7 +499,7 @@ LayoutObject* SVGUseElement::CreateLayoutObject(const ComputedStyle& style,
 static bool IsDirectReference(const SVGElement& element) {
   return IsSVGPathElement(element) || IsSVGRectElement(element) ||
          IsSVGCircleElement(element) || IsSVGEllipseElement(element) ||
-         IsSVGPolygonElement(element) || IsA<SVGPolylineElement>(element) ||
+         IsA<SVGPolygonElement>(element) || IsA<SVGPolylineElement>(element) ||
          IsSVGTextElement(element);
 }