Use new downcast helper for blink::SVGFEDiffuseLightingElement

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

Bug: 891908
Change-Id: I49b225ca6095376c1c72352569a845f02987e5c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936574
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720343}

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

@@ -67,7 +67,7 @@ void LayoutSVGResourceFilterPrimitive::StyleDidChange(
                         new_style.FloodColor());
     if (new_style.FloodOpacity() != old_style->SvgStyle().FloodOpacity())
       element.PrimitiveAttributeChanged(svg_names::kFloodOpacityAttr);
-  } else if (IsSVGFEDiffuseLightingElement(element) ||
+  } else if (IsA<SVGFEDiffuseLightingElement>(element) ||
              IsA<SVGFESpecularLightingElement>(element)) {
     CheckForColorChange(element, svg_names::kLightingColorAttr, diff,
                         old_style->SvgStyle().LightingColor(),

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

@@ -131,7 +131,7 @@ void SVGFELightElement::SvgAttributeChanged(const QualifiedName& attr_name) {
       return;
 
     SVGElement::InvalidationGuard invalidation_guard(this);
-    if (auto* diffuse = ToSVGFEDiffuseLightingElementOrNull(*parent))
+    if (auto* diffuse = DynamicTo<SVGFEDiffuseLightingElement>(*parent))
       diffuse->LightElementAttributeChanged(this, attr_name);
     else if (auto* specular = DynamicTo<SVGFESpecularLightingElement>(*parent))
       specular->LightElementAttributeChanged(this, attr_name);