Use new downcast helper for blink::SVGScriptElement

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

Bug: 891908
Change-Id: I74d59ca2549e7e19291f205b37b721e4eb3da5ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1936690Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#719614}

third_party/blink/renderer/core/html/parser/html_construction_site.cc

@@ -98,7 +98,7 @@ static bool HasImpliedEndTag(const HTMLStackItem* item) {
 
 static bool ShouldUseLengthLimit(const ContainerNode& node) {
   return !IsA<HTMLScriptElement>(node) && !IsA<HTMLStyleElement>(node) &&
-         !IsSVGScriptElement(node);
+         !IsA<SVGScriptElement>(node);
 }
 
 static unsigned TextLengthLimitForContainer(const ContainerNode& node) {

third_party/blink/renderer/core/script/script_element_base.cc

@@ -13,7 +13,7 @@ ScriptLoader* ScriptLoaderFromElement(Element* element) {
   ScriptLoader* script_loader = nullptr;
   if (auto* html_script = DynamicTo<HTMLScriptElement>(*element))
     script_loader = html_script->Loader();
-  else if (auto* svg_script = ToSVGScriptElementOrNull(*element))
+  else if (auto* svg_script = DynamicTo<SVGScriptElement>(*element))
     script_loader = svg_script->Loader();
   DCHECK(script_loader);
   return script_loader;

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

@@ -188,7 +188,7 @@ void SVGAnimateElement::ResolveTargetProperty() {
   // also disallows the perfectly "valid" animation of 'className' on said
   // element. If SVGScriptElement.href is transitioned off of SVGAnimatedHref,
   // this can be removed.
-  if (IsSVGScriptElement(*targetElement())) {
+  if (IsA<SVGScriptElement>(*targetElement())) {
     type_ = kAnimatedUnknown;
     css_property_id_ = CSSPropertyID::kInvalid;
   }