Use new downcast helper for blink::HTMLBaseElement

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

Bug: 891908
Change-Id: I7ba795ddbf3a0650a0ca212066d53328559d1e38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1728537Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Cr-Commit-Position: refs/heads/master@{#682739}

third_party/blink/renderer/core/editing/commands/replace_selection_command.cc

@@ -828,7 +828,7 @@ VisiblePosition ReplaceSelectionCommand::PositionAtStartOfInsertedContent()
 static void RemoveHeadContents(ReplacementFragment& fragment) {
   Node* next = nullptr;
   for (Node* node = fragment.FirstChild(); node; node = next) {
-    if (IsHTMLBaseElement(*node) || IsHTMLLinkElement(*node) ||
+    if (IsA<HTMLBaseElement>(*node) || IsHTMLLinkElement(*node) ||
         IsHTMLMetaElement(*node) || IsHTMLTitleElement(*node)) {
       next = NodeTraversal::NextSkippingChildren(*node);
       fragment.RemoveNode(node);

third_party/blink/renderer/core/frame/web_frame_serializer_impl.cc

@@ -160,7 +160,7 @@ String WebFrameSerializerImpl::PreActionBeforeSerializeOpenTag(
       // See http://msdn2.microsoft.com/en-us/library/ms537628(VS.85).aspx.
       result.Append(
           WebFrameSerializer::GenerateMarkOfTheWebDeclaration(param->url));
-    } else if (IsHTMLBaseElement(*element)) {
+    } else if (IsA<HTMLBaseElement>(*element)) {
       // Comment the BASE tag when serializing dom.
       result.Append("<!--");
     }
@@ -249,7 +249,7 @@ String WebFrameSerializerImpl::PostActionAfterSerializeEndTag(
   if (!param->is_html_document)
     return result.ToString();
   // Comment the BASE tag when serializing DOM.
-  if (IsHTMLBaseElement(*element)) {
+  if (IsA<HTMLBaseElement>(*element)) {
     result.Append("-->");
     // Append a new base tag declaration.
     result.Append(GenerateBaseTagDeclaration(param->document->BaseTarget()));