Use new downcast helper for blink::SVGPolylineElement

This CL has goal to use IsA<SVGPolylineElement>(element) in place of
IsSVGPolylineElement(element)

Bug: 891908
Change-Id: Icf9966502255b06ee97ea085374054d3498d4b05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936616Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#720116}

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) ||
-         IsSVGPolylineElement(target) || IsSVGPolygonElement(target) ||
+         IsA<SVGPolylineElement>(target) || IsSVGPolygonElement(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) || IsSVGPolylineElement(element) ||
+         IsSVGPolygonElement(element) || IsA<SVGPolylineElement>(element) ||
          IsSVGTextElement(element);
 }