Use new downcast helper for blink::HTMLHRElement

This CL has goal to use DynamicTo<HTMLHRElement> as new downcast helper

Bug: 891908
Change-Id: Icb20e0163d092e08e0bf71ecd75bfcfb140c6e27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1930539Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Abhijeet | Igalia <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#718998}

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

@@ -275,7 +275,7 @@ void InternalPopupMenu::WriteDocument(SharedBuffer* data) {
       AddOption(context, *option);
     else if (auto* optgroup = DynamicTo<HTMLOptGroupElement>(child))
       AddOptGroup(context, *optgroup);
-    else if (auto* hr = ToHTMLHRElementOrNull(child))
+    else if (auto* hr = DynamicTo<HTMLHRElement>(child))
       AddSeparator(context, *hr);
   }
   context.FinishGroupIfNecessary();
@@ -545,7 +545,7 @@ void InternalPopupMenu::Update() {
       AddOption(context, *option);
     else if (auto* optgroup = DynamicTo<HTMLOptGroupElement>(child))
       AddOptGroup(context, *optgroup);
-    else if (auto* hr = ToHTMLHRElementOrNull(child))
+    else if (auto* hr = DynamicTo<HTMLHRElement>(child))
       AddSeparator(context, *hr);
   }
   context.FinishGroupIfNecessary();