Use new downcast helper for blink::SVGTextPositioningElement

Bug: 891908
Change-Id: I1653b11914eb81117a4d9b35e2ef32ec38ed57bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1714949Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#680306}

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

@@ -106,8 +106,7 @@ static SVGTextPositioningElement* PositioningElementFromLayoutObject(
   DCHECK(node);
   DCHECK(node->IsSVGElement());
 
-  return IsSVGTextPositioningElement(*node) ? ToSVGTextPositioningElement(node)
-                                            : nullptr;
+  return DynamicTo<SVGTextPositioningElement>(node);
 }
 
 void SVGTextLayoutAttributesBuilder::CollectTextPositioningElements(

third_party/blink/renderer/core/svg/svg_text_positioning_element.h

@@ -57,7 +57,13 @@ inline bool IsSVGTextPositioningElement(const SVGElement& element) {
   return element.IsTextPositioning();
 }
 
-DEFINE_SVGELEMENT_TYPE_CASTS_WITH_FUNCTION(SVGTextPositioningElement);
+template <>
+struct DowncastTraits<SVGTextPositioningElement> {
+  static bool AllowFrom(const Node& node) {
+    auto* svg_element = DynamicTo<SVGElement>(node);
+    return svg_element && IsSVGTextPositioningElement(*svg_element);
+  }
+};
 
 }  // namespace blink