Use new downcast helper for blink::HTMLFieldSetElement

This CL has goal to use IsA<HTMLFieldSetElement>(element) in place of
IsHTMLFieldSetElement(element)

Bug: 891908
Change-Id: I7aa818e1cf1b75fe1c96c8a8e9fc781a32879a85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1930766Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#719000}

third_party/blink/renderer/core/html/forms/listed_element.cc

@@ -573,7 +573,7 @@ void ListedElement::UpdateAncestorDisabledState() const {
       last_legend_ancestor = ancestor;
       continue;
     }
-    if (!IsHTMLFieldSetElement(*ancestor))
+    if (!IsA<HTMLFieldSetElement>(*ancestor))
       continue;
     may_have_field_set_ancestor_ = true;
     if (ancestor->IsDisabledFormControl()) {