Use new downcast helper for blink::SVGFESpecularLightingElement

This CL has two goals,
  1. Use DynamicTo<SVGFESpecularLightingElement> as new downcast helper
  2. Use IsA<SVGFESpecularLightingElement>(element) in place of
     IsSVGFESpecularLightingElement(element)

Bug: 891908
Change-Id: I757fcb7e93004f35d71a3b570fed670fa3245db2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936583
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720270}

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

@@ -68,7 +68,7 @@ void LayoutSVGResourceFilterPrimitive::StyleDidChange(
     if (new_style.FloodOpacity() != old_style->SvgStyle().FloodOpacity())
       element.PrimitiveAttributeChanged(svg_names::kFloodOpacityAttr);
   } else if (IsSVGFEDiffuseLightingElement(element) ||
-             IsSVGFESpecularLightingElement(element)) {
+             IsA<SVGFESpecularLightingElement>(element)) {
     CheckForColorChange(element, svg_names::kLightingColorAttr, diff,
                         old_style->SvgStyle().LightingColor(),
                         new_style.LightingColor());

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

@@ -133,7 +133,7 @@ void SVGFELightElement::SvgAttributeChanged(const QualifiedName& attr_name) {
     SVGElement::InvalidationGuard invalidation_guard(this);
     if (auto* diffuse = ToSVGFEDiffuseLightingElementOrNull(*parent))
       diffuse->LightElementAttributeChanged(this, attr_name);
-    else if (auto* specular = ToSVGFESpecularLightingElementOrNull(*parent))
+    else if (auto* specular = DynamicTo<SVGFESpecularLightingElement>(*parent))
       specular->LightElementAttributeChanged(this, attr_name);
 
     return;