Use new downcast helper for blink::HTMLDirectoryElement

The primary goal of this CL is to use IsA<HTMLDirectoryElement>(element)
in place of IsHTMLDirectoryElement(element)

Bug: 891908
Change-Id: Iab622a1653b9b24d27894bf2ede0ba18a59daf61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1739487Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#684423}

third_party/blink/renderer/core/layout/layout_counter.cc

@@ -191,7 +191,7 @@ static bool PlanCounter(LayoutObject& object,
         return true;
       }
       if (IsHTMLUListElement(*e) || IsHTMLMenuElement(*e) ||
-          IsHTMLDirectoryElement(*e)) {
+          IsA<HTMLDirectoryElement>(*e)) {
         value = 0;
         is_reset = true;
         return true;