Use new downcast helper for blink::HTMLTemplateElement

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

Bug: 891908
Change-Id: I4220214482d1c4a78b3f5ef2cd450fabdf00c823
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1901150
Commit-Queue: Abhijeet Kandalkar <abhijeet@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713742}

third_party/blink/renderer/core/display_lock/display_lock_context_test.cc

@@ -961,7 +961,7 @@ TEST_F(DisplayLockContextTest, ElementInTemplate) {
   EXPECT_EQ(GetDocument().ActivationBlockingDisplayLockCount(), 0);
 
   auto* template_el =
-      ToHTMLTemplateElement(GetDocument().getElementById("template"));
+      To<HTMLTemplateElement>(GetDocument().getElementById("template"));
   auto* child = To<Element>(template_el->content()->firstChild());
   EXPECT_FALSE(child->isConnected());
 

third_party/blink/renderer/core/editing/testing/selection_sample.cc

@@ -41,9 +41,9 @@ void ConvertTemplatesToShadowRoots(HTMLElement& element) {
     Document* const document = element.ownerDocument();
     ShadowRoot& shadow_root =
         parent->AttachShadowRootInternal(ShadowRootType::kOpen);
-    Node* const fragment =
-        document->importNode(ToHTMLTemplateElement(template_element)->content(),
-                             true, ASSERT_NO_EXCEPTION);
+    Node* const fragment = document->importNode(
+        To<HTMLTemplateElement>(template_element)->content(), true,
+        ASSERT_NO_EXCEPTION);
     shadow_root.AppendChild(fragment);
   }
 }

third_party/blink/renderer/core/html/html_template_element.cc

@@ -60,8 +60,10 @@ void HTMLTemplateElement::CloneNonAttributePropertiesFrom(
     CloneChildrenFlag flag) {
   if (flag == CloneChildrenFlag::kSkip)
     return;
-  if (ToHTMLTemplateElement(source).content_)
-    content()->CloneChildNodesFrom(*ToHTMLTemplateElement(source).content());
+
+  auto& html_template_element = To<HTMLTemplateElement>(source);
+  if (html_template_element.content_)
+    content()->CloneChildNodesFrom(*html_template_element.content());
 }
 
 void HTMLTemplateElement::DidMoveToNewDocument(Document& old_document) {